Compiler: Fix asserting when returning a string literal.

Imported from SM: https://bugs.alliedmods.net/show_bug.cgi?id=3836.

compiler/libpc300/sc1.c

@@ -5251,6 +5251,11 @@ static void doreturn(void)
       error(78);                        /* mix "return;" and "return value;" */
     ident=doexpr(TRUE,FALSE,TRUE,TRUE,&tag,&sym,TRUE);
     needtoken(tTERM);
+	if (ident == iARRAY && sym == NULL) {
+		/* returning a literal string is not supported (it must be a variable) */
+		error(39);
+		ident = iCONSTEXPR;                 /* avoid handling an "array" case */
+	} /* if */
     /* see if this function already has a sub type (an array attached) */
     sub=finddepend(curfunc);
     assert(sub==NULL || sub->ident==iREFARRAY);