Misc minor items found in code.

Misc minor items found in code.

* Add some reentrancy protection in pbx.c when creating the contexts_table
hash table.

* Fix inverted test in chan_sip.c conditional code.

* Fix uninitialized variable and use of the wrong variable in chan_iax2.c.

* Fix test of return value in app_parkandannounce.c.  Explicitly testing
for -1 is bad if the function does not actually return that value when it
fails.

* Fixup some comments and add some curly braces in features.c.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@331248 65c4cc65-6c06-0410-ace0-fbb531ad65f3