RT-121512 - Allow -I/dir/ with trailing slash to find .pmc files.

6b0bdd7f2041803dc3ec72b53d28052705861967 updated -I to not add a
trailing '/' if one was already present, but failed to update the
length of the resulting SV to account for the lack of another character.
This caused later checks based off of that length to fail (in this case,
seeing if the last 3 characters of the string are .pm).
Missed perldelta for 437e3a7 (for v5.19.8)