src/profiler.c: Try and fix occasional assertion failures

src/profiler.c: Try and fix occasional assertion failures

Apparently the (>= match imatch) test fails sometimes in `profiler.el`.
Not sure where this comes from, but this patch should remove one
possible source.

* src/profiler.c (Fprofiler_cpu_log, Fprofiler_memory_log):
Change the special Automatic_GC backtrace to make it clear that it's
a *sibling* of the call tree (i.e. it's at the (its own) root).
(malloc_probe): Obey `size` when incrementing the gc_counter.