COMMON: Restore old behaviour for appending char to string

Appending \0 to string and expecting it to be just dropped is
still an invalid behaviour but it already happened in 2 engines, so
restore old behaviour, at least for now

common/base-str.cpp

@@ -731,6 +731,8 @@ TEMPLATE void BASESTRING::trim() {
 #endif
 
 TEMPLATE void BASESTRING::assignAppend(value_type c) {
+	if (c == 0)
+		return;
 	ensureCapacity(_size + 1, true);
 
 	_str[_size++] = c;