get_matching_bracket bail early if instructions struct too small

[rwhitworth]

Avoids this valgrind output:

==16650== Process terminating with default action of signal 11 (SIGSEGV): dumping core
==16650==  Bad permissions for mapped region at address 0x403B000
==16650==    at 0x4037CA: get_matching_bracket (helpers.c:75)
==16650==    by 0x405C17: optimize_multiplication (optimize.c:94)
==16650==    by 0x401BCB: main (yabfc.c:137)

[julianneswinoga]

What input file are you running into this error with?

[rwhitworth]

input.gz