fix incorrect comparison loop condition in memmem

fix incorrect comparison loop condition in memmem

the logic for this loop was copied from null-terminated-string logic
in strstr without properly adapting it to work with explicit lengths.

presumably this error could result in false negatives (wrongly
comparing past the end of the needle/haystack), false positives
(stopping comparison early when the needle contains null bytes), and
crashes (from runaway reads past the end of mapped memory).