main/pbx.c: Fix double lock of contexts lock introduced by r429967

We only need to hold the context_merge_lock once. Locking it twice will make
many other parts of Asterisk very sad.

ASTERISK-24641 #close


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@430111 f38db490-d61c-443f-a65b-d21fe96a405b