lib/khash: when resizing to 0, free, don't realloc
lib/khash: when resizing to 0, free, don't realloc

Caught by the Clang Static Analyzer. Better safe than sorry here since
I'm not familiar enough with the code to be sure that having allocated
but size-0 tables would be okay.

Deewiant/mushspace,Deewiant/mushspace,Deewiant/mushspace