Avoid a segfault on invalid format names

If a format name was not found by ast_getformatbyname, a NULL pointer
would be passed into ast_format_rate and immediately dereferenced.
This ensures that a valid pointer is used since the structure is
already allocated on the stack.

(closes issue DPH-523)
Reported-by: Steve Pitts
........

Merged revisions 374932 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@374933 65c4cc65-6c06-0410-ace0-fbb531ad65f3