The original sort_by_src_len() function returned 0 even when a > b,
violating the C standard requirements for qsort() comparison functions.
Specifically, it broke antisymmetry and transitivity, which can result
in undefined behavior.
In some versions of glibc, this leads not only to incorrect sorting but
also potential memory corruption[1].
Fix the issue by returning -1 when a > b, restoring compliance with the
standard.
Link: https://www.qualys.com/2024/01/30/qsort.txt [1]
Fixes: 8eb2558a7 ("Add -m 6211 module and moved some code around")
