awk: Fix subobject out-of-bounds access

[arichardson]

Summary

When matching a regex with ^, it would attempt to access
gototab[NSTATES][NCHARS+2], and therefore access the state for the \002
character instead. This change is required to run awk under CHERI (with
sub-object bounds) and when running with UBSAN instrumentation.

This was committed upstream as https://github.com/onetrueawk/awk/commit/cbf924342b63a095a4c6842280c3085b1b63ae45

Found by: CHERI (with subobject bounds enabled)
Obtained from: CheriBSD