backport tim_one's checkin of

backport tim_one's checkin of
    revision 2.38 of mmapmodule.c

SF bug 515943:  searching for data with \0 in mmap.
mmap_find_method():  this obtained the string to find via s#, but it
ignored its length, acting as if it were \0-terminated instead.

Someone please run on Linux too (the extended test_mmap works on Windows).

Bugfix candidate.