BugFix: return an empty type name if retrieve_type_dbg is NULL

This fixes a Windows crash.

[r27796]

ir/debug/dbginfo.c

@@ -127,10 +127,11 @@ void ir_set_type_debug_retrieve(retrieve_type_dbg_func func)
 void ir_retrieve_type_dbg_info(char *buffer, size_t buffer_size,
                                const type_dbg_info *tdbgi)
 {
+	buffer[0] = '\0';
 	if (retrieve_type_dbg)
 		retrieve_type_dbg(buffer, buffer_size, tdbgi);
 	assert(buffer_size > 0);
-	buffer[buffer_size-1] = 0;
+	buffer[buffer_size-1] = '\0';
 }
 
 void ir_dbg_info_snprint(char *buf, size_t bufsize, const dbg_info *dbg)