[krbdev.mit.edu #5948] error in filebase+suffix list generation in plugin code

[Ken Raeburn via RT]

krb5int_get_plugin_filenames works from two lists, filebases[] and fileexts[], to build a list of 
filenames to look for.  However, it's allocating M+N elements in its array, computing all M*N 
combinations, and sticking them into the M+N slots in overlapping passes.  No overrun here, 
but we can leak memory and omit some filenames from the final list, if filebases ever gets more 
than one element, which I think it currently does not.