py/gc: In gc_alloc, reset n_free var right before search for free mem.

Otherwise there is the possibility that n_free starts out non-zero from the
previous iteration, which may have found a few (but not enough) free blocks
at the end of the heap.  If this is the case, and if the very first blocks
that are scanned the second time around (starting at
gc_last_free_atb_index) are found to give enough memory (including the
blocks at the end of the heap from the previous iteration that left n_free
non-zero) then memory will be allocated starting before the location that
gc_last_free_atb_index points to, most likely leading to corruption.

This serious bug did not manifest itself in the past because a gc_collect
always resets gc_last_free_atb_index to point to the start of the GC heap,
and the first block there is almost always allocated to a long-lived
object (eg entries from sys.path, or mounted filesystem objects), which
means that n_free would be reset at the start of the search loop.

But with threading enabled with the GIL disabled it is possible to trigger
the bug via the following sequence of events:

1. Thread A runs gc_alloc, fails to find enough memory, and has a non-zero
   n_free at the end of the search.
2. Thread A calls gc_collect and frees a bunch of blocks on the GC heap.
3. Just after gc_collect finishes in thread A, thread B takes gc_mutex and
   does an allocation, moving gc_last_free_atb_index to point to the
   interior of the heap, to a place where there is most likely a run of
   available blocks.
4. Thread A regains gc_mutex and does its second search for free memory,
   starting with a non-zero n_free.  Since it's likely that the first block
   it searches is available it will allocate memory which overlaps with the
   memory before gc_last_free_atb_index.

py/gc.c

@@ -453,7 +453,7 @@ void *gc_alloc(size_t n_bytes, bool has_finaliser) {
     size_t i;
     size_t end_block;
     size_t start_block;
-    size_t n_free = 0;
+    size_t n_free;
     int collected = !MP_STATE_MEM(gc_auto_collect_enabled);
 
     #if MICROPY_GC_ALLOC_THRESHOLD
@@ -468,6 +468,7 @@ void *gc_alloc(size_t n_bytes, bool has_finaliser) {
     for (;;) {
 
         // look for a run of n_blocks available blocks
+        n_free = 0;
         for (i = MP_STATE_MEM(gc_last_free_atb_index); i < MP_STATE_MEM(gc_alloc_table_byte_len); i++) {
             byte a = MP_STATE_MEM(gc_alloc_table_start)[i];
             if (ATB_0_IS_FREE(a)) { if (++n_free >= n_blocks) { i = i * BLOCKS_PER_ATB + 0; goto found; } } else { n_free = 0; }