py/gc: When GC threshold is hit don't unnecessarily collect twice.

Without this, if GC threshold is hit and there is not enough memory left to
satisfy the request, gc_collect() will run a second time and the search for
memory will happen again and will fail again.

Thanks to @adritium for pointing out this issue, see #3786.

py/gc.c

@@ -453,6 +453,7 @@ void *gc_alloc(size_t n_bytes, bool has_finaliser) {
     if (!collected && MP_STATE_MEM(gc_alloc_amount) >= MP_STATE_MEM(gc_alloc_threshold)) {
         GC_EXIT();
         gc_collect();
+        collected = 1;
         GC_ENTER();
     }
     #endif