Check if parent exists for local label reference

If an attempt is made to reference a local label before any non-local label
is defined, raise an error instead of segfaulting.

src/asm/symbol.c

@@ -735,6 +735,9 @@ void sym_Ref(char *tzSym)
 		int isLocal = 0;
 
 		if (*tzSym == '.') {
+			if (!pScope)
+				fatalerror("Local label reference '%s' in main scope",
+					   tzSym);
 			fullSymbolName(fullname, sizeof(fullname), tzSym,
 				       pScope);
 			tzSym = fullname;