py/gc: Refactor assertions in gc_free function.

gc_free() expects either NULL or a valid pointer into the heap, so the
checks for a valid pointer can be turned into assertions.

py/gc.c

@@ -536,12 +536,18 @@ void gc_free(void *ptr) {
 
     DEBUG_printf("gc_free(%p)\n", ptr);
 
-    if (VERIFY_PTR(ptr)) {
+    if (ptr == NULL) {
+        GC_EXIT();
+    } else {
+        // get the GC block number corresponding to this pointer
+        assert(VERIFY_PTR(ptr));
         size_t block = BLOCK_FROM_PTR(ptr);
-        if (ATB_GET_KIND(block) == AT_HEAD) {
+        assert(ATB_GET_KIND(block) == AT_HEAD);
+
         #if MICROPY_ENABLE_FINALISER
         FTB_CLEAR(block);
         #endif
+
         // set the last_free pointer to this block if it's earlier in the heap
         if (block / BLOCKS_PER_ATB < MP_STATE_MEM(gc_last_free_atb_index)) {
             MP_STATE_MEM(gc_last_free_atb_index) = block / BLOCKS_PER_ATB;
@@ -558,15 +564,6 @@ void gc_free(void *ptr) {
         #if EXTENSIVE_HEAP_PROFILING
         gc_dump_alloc_table();
         #endif
-        } else {
-            GC_EXIT();
-            assert(!"bad free");
-        }
-    } else if (ptr != NULL) {
-        GC_EXIT();
-        assert(!"bad free");
-    } else {
-        GC_EXIT();
     }
 }