proc/sig.c: Fix the strtosig() function.

proc/sig.c: Fix the strtosig() function.

Do not memleak "copy" in case of an error.

Do not use "sizeof(converted)" in snprintf(), since "converted" is a
"char *" (luckily, 8 >= sizeof(char *)). Also, remove "sizeof(char)"
which is guaranteed to be 1 by the C standard, and replace 8 with 12,
which is enough to hold any stringified int and does not consume more
memory (in both cases, the glibc malloc()ates a minimum-sized chunk).