BUG/MEDIUM: signal: signal handler does not properly check for signal bounds

BUG/MEDIUM: signal: signal handler does not properly check for signal bounds

sig is checked for < 0 or > MAX_SIGNAL, but the signal array is
MAX_SIGNAL in size. At the moment, MAX_SIGNAL is 256. If a system supports
more than MAX_SIGNAL signals, then sending signal MAX_SIGNAL to the process
will corrupt one integer in its memory and might also crash the process.

This bug is also present in 1.4 and 1.3, and the fix must be backported.

Reported-by: Dinko Korunic <dkorunic@reflected.net>