Eliminate GCC warning about uninitialized 'hhdr' in GC_allochblk_nth

* allchblk.c (GC_allochblk_nth): Replace "for" condition with
conditional "return" since otherwise GCC reports
"'hhdr' may be used uninitialized" warning in case of -O0 optimization
level; remove comment about bogus compiler warning; refine comment
for "thishdr" local variable.