Fix bug in GC_CHECK_MARKED_OBJECTS check

Fix bug in GC_CHECK_MARKED_OBJECTS check

* src/alloc.c (mark_object): Fix bug in checking code.
When GC_CHECK_MARKED_OBJECTS is defined, the bug caused
CHECK_ALLOCATED_AND_LIVE_SYMBOL to repeatedly do the
CHECK_ALLOCATED and CHECK_LIVE tests for the first symbol in each
bucket.  The bug did not affect behavior either in the normal case
where GC_CHECK_MARKED_OBJECTS is not defined, or where Emacs does
not have an internal error that a properly-written
CHECK_ALLOCATED_AND_LIVE_SYMBOL would detect.