clang compiler warnings: Fix -Wself-assign

Assigning a variable to itself isn't super useful. However, the WAV format
modules make use of this in order to perform byte endian checks. This patch
works around the warning by only performing the self assignment if we are
going to do more than just assign it to ourselves. Which is odd, but true.

Review: https://reviewboard.asterisk.org/r/4544/

ASTERISK-24917
Reported by: dkdegroot
patches:
  rb4544.patch submitted by dkdegroot (License 6600)
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Merged revisions 433690 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 433691 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@433692 65c4cc65-6c06-0410-ace0-fbb531ad65f3