main/untils: Prevent potential infinite loop in ast_careful_fwrite

A loop in ast_careful_fwrite exists that will continually attempt to write to
a file stream, even in the presence of EAGAIN/EINTR errors. However, if a
connection that uses ast_careful_fwrite closes suddenly, ast_careful_fwrite's
call to fflush may return EAGAIN/EINTER along with EOF. A subsequent call to
fflush will return EOF but not clear errno, resulting in an infinite loop.

This patch clears errno after it is detected and handled the loop, such that
any subsequent call to fflush will not get erroneously stuck.

Review: https://reviewboard.asterisk.org/r/3704

#ASTERISK-23984 #close
Reported by: Steve Davies
patches:
  fflush_loop_fix uploaded by one47 (License 5012)
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Merged revisions 417797 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 417798 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@417799 f38db490-d61c-443f-a65b-d21fe96a405b