Fix file descriptor leak and pointer scope issue in MiniVM when sending mail

When MiniVM sends an e-mail and it has the volgain option set, it will spawn
sox in a separate process to handle the manipulation of the sound file.  In
doing so, it creates a temporary file.  There are two problems here:
  1) The file descriptor returned from mkstemp is leaked
  2) The finalfilename character pointer points to a buffer that loses scope
     once volgain processing is finished.

Note that in r316265, Russell fixed some gcc warnings by using the return
value of the mkstemp call.  A warning was placed in minivm that the file
descriptor was going to be leaked.  This patch reverts that change, as it
handles the leak and 'uses' the file descriptor returned from mkstemp.

(closes issue ASTERISK-17133)
Reported by: Tzafrir Cohen
patches:
  minivm_18501_demo.diff uploaded by Tzafrir Cohen (license #5035)
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Merged revisions 372554 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 372555 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 372556 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@372557 65c4cc65-6c06-0410-ace0-fbb531ad65f3