MFC r287148: sh: Fix out of bounds read when there is no ] after a [:class:].

MFC r287148: 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.

git-svn-id: svn://svn.freebsd.org/base/stable/10@287752 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f