Fix NULL pointer segfault in ast_sockaddr_parse()

While working with ast_parse_arg() to perform a validity check, a segfault
occurred.  The segfault occurred due to passing a NULL pointer to
ast_sockaddr_parse() from ast_parse_arg().  According to the documentation in
config.h, "result pointer to the result.  NULL is valid here, and can be used to
perform only the validity checks."

This patch fixes the segfault by checking for a NULL pointer.  This patch also
adds documentation to netsock2.h about why it is necessary to check for a NULL
pointer.

(Closes issue ASTERISK-20006)
Reported by: Michael L. Young
Tested by: Michael L. Young
Patches:
asterisk-20006-netsock-null-ptr.diff uploaded by Michael L. Young (license 5026)

Review: https://reviewboard.asterisk.org/r/1990/
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Merged revisions 369108 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 369109 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369110 65c4cc65-6c06-0410-ace0-fbb531ad65f3