XcursorFileSaveImages: plug memory leak on invalid input

Error: Memory leak (CWE 401)
   Memory leak of pointer 'comments' allocated with XcursorCommentsCreate(0)
        at line 982 of src/file.c in function 'XcursorFileSaveImages'.
          'comments' allocated at line 978 with XcursorCommentsCreate(0).
          comments leaks when comments != 0 at line 981.

[ This bug was found by the Parfait 0.3.7 bug checking tool.
  For more information see http://labs.oracle.com/projects/parfait/

 ]

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Jeremy Huddleston <jeremyhu@apple.com>