Return proper error message for ill formed variable reference

[alamb]

Which issue does this PR close?

Closes #3137 . Kudos to @syheliel for the detailed error report and reproducer 👍

Rationale for this change

Errors are better than panics

What changes are included in this PR?

Check length before slicing 2

Are there any user-facing changes?

Better error message

[codecov-commenter]

Codecov Report

Merging #3162 (f6eb7e3) into master (5778ef2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head f6eb7e3 differs from pull request most recent head 9878ad0. Consider uploading reports for the commit 9878ad0 to get more accurate results

@@           Coverage Diff           @@
##           master    #3162   +/-   ##
=======================================
  Coverage   85.87%   85.88%           
=======================================
  Files         291      291           
  Lines       52758    52765    +7     
=======================================
+ Hits        45307    45316    +9     
+ Misses       7451     7449    -2     

Impacted Files	Coverage Δ
datafusion/core/src/execution/context.rs	78.24% <100.00%> (+0.18%)	⬆️
datafusion/core/src/physical_plan/metrics/value.rs	86.93% <0.00%> (-0.51%)	⬇️
datafusion/sql/src/planner.rs	82.05% <0.00%> (+0.04%)	⬆️
datafusion/common/src/scalar.rs	84.61% <0.00%> (+0.06%)	⬆️
datafusion/expr/src/logical_plan/plan.rs	77.60% <0.00%> (+0.17%)	⬆️

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[ursabot]

Benchmark runs are scheduled for baseline = 89bcfc4 and contender = f677e50. f677e50 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java