sh: Fix out of bounds read when there is no ] after a [:class:].

sh: Fix out of bounds read when there is no ] after a [:class:].

The initial check for a matching ] was incorrect if a ] may be consumed by a
[:class:]. The subsequent loop assumed that there must be a ].

Remove the initial check and make the loop cope with a missing ].

Found with afl-fuzz.

MFC after:	1 week