eina hash - fix stringshare key comparison function

eina hash - fix stringshare key comparison function

this fixes T3638

@fix

a note... thanks so much to aerodynamik for spotting this. i'm rather
surprised coverity didn't spot this... unless someone said to "shut up
coverity you're wrong" and they should not have.

i also might have expected compilers to spot this too... and add a
warning.

anyway ... this was a seriously subtle bug that could have caused all
kinds of havoc in efl.  keys that are different may be compared to be
the same. it could get ordering wrong and sorting thus maybe insert
keys that cannot be found anymore and oh so much more besides.