Don't assume that rangeOfCharacterFromSet:options: always returns a length of...

Don't assume that rangeOfCharacterFromSet:options: always returns a length of 1. This fixes a bug where copying text where a line ends in a surrogate pair fails because it creates an invalid UTF-16 string, chopping the surrogate pair in half. Issue 3544.

PTYTextView.m

@@ -4049,7 +4049,7 @@ static double EuclideanDistance(NSPoint p1, NSPoint p2) {
     if (rangeOfLastWantedCharacter.location == NSNotFound) {
         [string deleteCharactersInRange:NSMakeRange(0, string.length)];
     } else if (rangeOfLastWantedCharacter.location < string.length - 1) {
-        NSUInteger i = rangeOfLastWantedCharacter.location + 1;
+        NSUInteger i = rangeOfLastWantedCharacter.location + rangeOfLastWantedCharacter.length;
         [string deleteCharactersInRange:NSMakeRange(i, string.length - i)];
     }
 }