Disable ~X when message scoring. (closes #3861)

Disable ~X when message scoring.  (closes #3861)

mutt_score_message() purposely passes a NULL context to
mutt_pattern_exec().  The idea was to block slow patterns, and the
scoring documentation notes this by saying:

  "For efficiency reasons, patterns which scan information not
  available in the index, such as ~b, ~B or ~h, may not be used"

~X needs the context to parse the messages (during message scoring at
least), and thus isn't suitable for message scoring either.

Block ~X from being used when the context is NULL.  Add ~X to the list
of patterns noted as unusable in the message scoring documentation.