linux PIString fix

libs/main/core/pistring.cpp

@@ -223,7 +223,7 @@ void PIString::appendFromChars(const char * c, int s, const char * codepage) {
 	}
 #else
 #  ifdef WINDOWS
-	if (s == -1) s = strlen(s);
+	if (s == -1) s = strlen(c);
 	int sz = MultiByteToWideChar((uint)(uintptr_t)codepage, MB_ERR_INVALID_CHARS, c, s, 0, 0);
 	if (sz <= 0) return;
 	enlarge(sz);