Decode: don't crash on embedded nil pointers

[pelletier]

Also has the perks of reducing the overhead of FindByIndex:

name                                old time/op    new time/op    delta
UnmarshalDataset/config-32            17.0ms ± 1%    17.0ms ± 1%    ~     (p=1.000 n=5+5)
UnmarshalDataset/canada-32            71.6ms ± 1%    71.4ms ± 1%    ~     (p=1.000 n=5+5)
UnmarshalDataset/citm_catalog-32      24.2ms ± 3%    23.5ms ± 2%  -3.03%  (p=0.032 n=5+5)
UnmarshalDataset/twitter-32           9.37ms ± 1%    9.09ms ± 2%  -2.97%  (p=0.032 n=5+5)
UnmarshalDataset/code-32              75.4ms ± 2%    74.9ms ± 0%    ~     (p=0.222 n=5+5)
UnmarshalDataset/example-32            147µs ±10%     136µs ± 1%  -7.14%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/struct-32     512ns ± 2%     500ns ± 0%  -2.35%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/map-32        721ns ± 2%     702ns ± 1%  -2.68%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/struct-32     40.1µs ± 0%    39.6µs ± 0%  -1.30%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/map-32        62.3µs ± 1%    60.6µs ± 0%  -2.83%  (p=0.008 n=5+5)
Unmarshal/HugoFrontMatter-32          10.8µs ± 1%    10.5µs ± 1%  -2.86%  (p=0.008 n=5+5)

name                                old speed      new speed      delta
UnmarshalDataset/config-32          61.8MB/s ± 1%  61.8MB/s ± 1%    ~     (p=1.000 n=5+5)
UnmarshalDataset/canada-32          30.8MB/s ± 1%  30.8MB/s ± 1%    ~     (p=1.000 n=5+5)
UnmarshalDataset/citm_catalog-32    23.0MB/s ± 3%  23.8MB/s ± 2%  +3.09%  (p=0.032 n=5+5)
UnmarshalDataset/twitter-32         47.2MB/s ± 1%  48.6MB/s ± 2%  +3.09%  (p=0.032 n=5+5)
UnmarshalDataset/code-32            35.6MB/s ± 2%  35.9MB/s ± 0%    ~     (p=0.222 n=5+5)
UnmarshalDataset/example-32         55.3MB/s ±10%  59.4MB/s ± 1%  +7.36%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/struct-32  21.5MB/s ± 2%  22.0MB/s ± 0%  +2.41%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/map-32     15.2MB/s ± 2%  15.7MB/s ± 1%  +2.74%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/struct-32    131MB/s ± 0%   132MB/s ± 0%  +1.31%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/map-32      84.1MB/s ± 1%  86.6MB/s ± 0%  +2.91%  (p=0.008 n=5+5)
Unmarshal/HugoFrontMatter-32        50.6MB/s ± 1%  52.1MB/s ± 1%  +2.93%  (p=0.008 n=5+5)

name                                old alloc/op   new alloc/op   delta
UnmarshalDataset/config-32            5.86MB ± 0%    5.86MB ± 0%    ~     (p=0.579 n=5+5)
UnmarshalDataset/canada-32            83.0MB ± 0%    83.0MB ± 0%    ~     (p=0.651 n=5+5)
UnmarshalDataset/citm_catalog-32      34.7MB ± 0%    34.7MB ± 0%    ~     (p=0.548 n=5+5)
UnmarshalDataset/twitter-32           12.7MB ± 0%    12.7MB ± 0%    ~     (p=1.000 n=5+5)
UnmarshalDataset/code-32              22.2MB ± 0%    22.2MB ± 0%    ~     (p=0.841 n=5+5)
UnmarshalDataset/example-32            186kB ± 0%     186kB ± 0%    ~     (p=0.111 n=5+5)
Unmarshal/SimpleDocument/struct-32      805B ± 0%      805B ± 0%    ~     (all equal)
Unmarshal/SimpleDocument/map-32       1.13kB ± 0%    1.13kB ± 0%    ~     (all equal)
Unmarshal/ReferenceFile/struct-32     20.9kB ± 0%    20.9kB ± 0%    ~     (p=0.643 n=5+5)
Unmarshal/ReferenceFile/map-32        38.3kB ± 0%    38.3kB ± 0%    ~     (p=0.397 n=5+5)
Unmarshal/HugoFrontMatter-32          7.44kB ± 0%    7.44kB ± 0%    ~     (all equal)

name                                old allocs/op  new allocs/op  delta
UnmarshalDataset/config-32              227k ± 0%      227k ± 0%    ~     (p=1.000 n=5+5)
UnmarshalDataset/canada-32              782k ± 0%      782k ± 0%    ~     (all equal)
UnmarshalDataset/citm_catalog-32        192k ± 0%      192k ± 0%    ~     (p=0.968 n=4+5)
UnmarshalDataset/twitter-32            56.9k ± 0%     56.9k ± 0%    ~     (p=0.429 n=4+5)
UnmarshalDataset/code-32               1.05M ± 0%     1.05M ± 0%    ~     (p=0.556 n=4+5)
UnmarshalDataset/example-32            1.36k ± 0%     1.36k ± 0%    ~     (all equal)
Unmarshal/SimpleDocument/struct-32      9.00 ± 0%      9.00 ± 0%    ~     (all equal)
Unmarshal/SimpleDocument/map-32         13.0 ± 0%      13.0 ± 0%    ~     (all equal)
Unmarshal/ReferenceFile/struct-32        183 ± 0%       183 ± 0%    ~     (all equal)
Unmarshal/ReferenceFile/map-32           642 ± 0%       642 ± 0%    ~     (all equal)
Unmarshal/HugoFrontMatter-32             141 ± 0%       141 ± 0%    ~     (all equal)

Fixes #807