Fix Shift-O on the last line of the file

This fixes an issue where, when the start of the line is the last
character of the file, there an off by 1 error that would cause shift-o
not to work. `characterAtIndex` expects and index within the bounds of
this string, the passed character index was previously the length of the
string.

This fixes XVimProject#675

XVim/NSTextStorage+VimOperation.m

@@ -249,7 +249,7 @@ - (NSUInteger)xvim_firstOfLine:(NSUInteger)index
 {
     NSUInteger pos = [self xvim_startOfLine:index];
 
-    if (pos == index && isNewline([self.xvim_string characterAtIndex:pos])) {
+    if (pos == index && isNewline([self.xvim_string characterAtIndex:(pos - 1)])) {
         return NSNotFound;
     }
     return pos;

XVim/NSTextView+VimOperation.m

@@ -380,7 +380,7 @@ - (void)_xvim_killSelection:(XVimSelection)sel
  **/
 
 - (NSUInteger)insertionPoint{
-    id ret = [self dataForName:@"insertionPoint"];
+    NSNumber* ret = [self dataForName:@"insertionPoint"];
     return nil == ret ? 0 : [ret unsignedIntegerValue];
 }