Fix undefined behaviors in `strings.c`

[ncruces]

strncmp can currently access beyond the end of string, as it assumes it can always read n chars.

memchr does not respect this sentence in the standard:

The implementation shall behave as if it reads the characters sequentially and stops as soon as a matching character is found.

For both the problem is intrinsically the same: they attempt to tolerate unaligned access, which may cross a page boundary into "unmapped" memory (or actually wrap around the whole 4GB).