Fix crash in Objective C

Bug (internal to Google): b/124992541

Author: Neil Fraser

objectivec/DiffMatchPatch.m

@@ -1598,8 +1598,8 @@ - (void)diff_cleanupSemantic:(NSMutableArray *)diffs;
  thisPointer = 1;
  while (thisPointer < diffs.count) {
  if (prevDiff.operation == DIFF_DELETE && thisDiff.operation == DIFF_INSERT) {
- NSString *deletion = prevDiff.text;
- NSString *insertion = thisDiff.text;
+ NSString *deletion = [prevDiff.text copy];
+ NSString *insertion = [thisDiff.text copy];
  NSUInteger overlap_length1 = (NSUInteger)diff_commonOverlap((CFStringRef)deletion, (CFStringRef)insertion);
  NSUInteger overlap_length2 = (NSUInteger)diff_commonOverlap((CFStringRef)insertion, (CFStringRef)deletion);
  if (overlap_length1 >= overlap_length2) {
@@ -1629,6 +1629,8 @@ - (void)diff_cleanupSemantic:(NSMutableArray *)diffs;
  thisPointer++;
  }
  }
+ [deletion release];
+ [insertion release];
  thisPointer++;
  }
  thisPointer++;