fix kv builder out of bounds panic on single backslash

[ccapurso]

The KV Builder panics when attempting to process a single backslash. The KV Builder add method currently assumes that the provided string has at least two characters when attempting to check for an escaped @ (i.e. \@). The proposed fix is to guard with a length check.

In additional to the added unit test, this change was functionally tested with the Vault kv secret engine CLI.

Before change:

❯ vault kv put secret/foo 'bar=\'
panic: runtime error: index out of range [1] with length 1

goroutine 1 [running]:
github.com/hashicorp/go-secure-stdlib/kv-builder.(*Builder).add(0xc0008cb440, {0x7ffeefbff898, 0x11880ee8})
        /Users/ccapurso/go/pkg/mod/github.com/hashicorp/go-secure-stdlib/kv-builder@v0.1.1/builder.go:94 +0x832
github.com/hashicorp/go-secure-stdlib/kv-builder.(*Builder).Add(0x7ffeefbff88d, {0xc0001a6040, 0x1, 0x484dde5})
        /Users/ccapurso/go/pkg/mod/github.com/hashicorp/go-secure-stdlib/kv-builder@v0.1.1/builder.go:32 +0x89
github.com/hashicorp/vault/command.parseArgsData({0x6449e40, 0xc000194000}, {0xc0001a6040, 0x1, 0x1})
        /Users/ccapurso/git/vault/command/base_helpers.go:134 +0xaa
github.com/hashicorp/vault/command.(*KVPutCommand).Run(0xc0008cab20, {0xc0001a6030, 0x2, 0x2})
        /Users/ccapurso/git/vault/command/kv_put.go:120 +0x171
github.com/mitchellh/cli.(*CLI).Run(0xc000145cc0)
        /Users/ccapurso/go/pkg/mod/github.com/mitchellh/cli@v1.1.2/cli.go:262 +0x5f8
github.com/hashicorp/vault/command.RunCustom({0xc0001a6010, 0x4, 0x4}, 0xc0000001a0)
        /Users/ccapurso/git/vault/command/main.go:180 +0x9f7
github.com/hashicorp/vault/command.Run(...)
        /Users/ccapurso/git/vault/command/main.go:88
main.main()
        /Users/ccapurso/git/vault/main.go:10 +0x52

After change:

❯ vault kv put secret/foo 'bar=\'
= Secret Path =
secret/data/foo

======= Metadata =======
Key                Value
---                -----
created_time       2022-03-15T14:38:07.024316Z
custom_metadata    <nil>
deletion_time      n/a
destroyed          false
version            1

❯ vault kv get secret/foo
= Secret Path =
secret/data/foo

======= Metadata =======
Key                Value
---                -----
created_time       2022-03-15T14:38:07.024316Z
custom_metadata    <nil>
deletion_time      n/a
destroyed          false
version            1

=== Data ===
Key    Value
---    -----
bar    \