Cleanup for pull-up-isReset patch.

Cleanup for pull-up-isReset patch.

Clear isReset before, not after, calling the context-specific alloc method,
so as to preserve the option to do a tail call in MemoryContextAlloc
(and also so this code isn't assuming that a failed alloc call won't have
changed the context's state before failing).  Fix missed direct invocation
of reset method.  Reformat a comment.