py/emitbc: Avoid undefined behavior calling memset() with NULL 1st arg.

Calling memset(NULL, value, 0) is not standards compliant so we must add an
explicit check that emit->label_offsets is indeed not NULL before calling
memset (this pointer will be NULL on the first pass of the parse tree and
it's more logical / safer to check this pointer rather than check that the
pass is not the first one).

Code sanitizers will warn if NULL is passed as the first value to memset,
and compilers may optimise the code based on the knowledge that any pointer
passed to memset is guaranteed not to be NULL.

py/emitbc.c

@@ -315,7 +315,7 @@ void mp_emit_bc_start_pass(emit_t *emit, pass_kind_t pass, scope_t *scope) {
     emit->last_source_line = 1;
     #ifndef NDEBUG
     // With debugging enabled labels are checked for unique assignment
-    if (pass < MP_PASS_EMIT) {
+    if (pass < MP_PASS_EMIT && emit->label_offsets != NULL) {
         memset(emit->label_offsets, -1, emit->max_num_labels * sizeof(mp_uint_t));
     }
     #endif