fix warning: comparison of unsigned expression < 0 is always false

teensy3/WString.cpp

 
 int String::lastIndexOf(char ch, unsigned int fromIndex) const
 {
-	if (fromIndex >= len || fromIndex < 0) return -1;
+	if (fromIndex >= len) return -1;
 	char tempchar = buffer[fromIndex + 1];
 	buffer[fromIndex + 1] = '\0';
 	char* temp = strrchr( buffer, ch );
 
 int String::lastIndexOf(const String &s2, unsigned int fromIndex) const
 {
-  	if (s2.len == 0 || len == 0 || s2.len > len || fromIndex < 0) return -1;
+  	if (s2.len == 0 || len == 0 || s2.len > len) return -1;
 	if (fromIndex >= len) fromIndex = len - 1;
 	int found = -1;
 	for (char *p = buffer; p <= buffer + fromIndex; p++) {