The bug report pointed out a bogosity in the comment block explaining
thread safety for arena management.  Repaired that comment, repaired a
couple others while I was at it, and added an assert.

_PyMalloc_DebugRealloc:  If this needed to get more memory, but couldn't,
it erroneously freed the original memory.  Repaired that.

This is for 2.3 only (unless we decide to backport the new pymalloc).