AST-2013-001: Prevent buffer overflow through H.264 format negotiation

The format attribute resource for H.264 video performs an unsafe read against a
media attribute when parsing the SDP. The value passed in with the format
attribute is not checked for its length when parsed into a fixed length buffer.
This patch resolves the vulnerability by only reading as many characters from
the SDP value as will fit into the buffer.

(closes issue ASTERISK-20901)
Reported by: Ulf Harnhammar
patches:
  h264_overflow_security_patch.diff uploaded by jrose (License 6182)
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Merged revisions 383973 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@383975 65c4cc65-6c06-0410-ace0-fbb531ad65f3