res_fax_spandsp: Fix crash when passing ulaw/alaw data to spandsp

When acting as a T.38 fax gateway, res_fax_spandsp would at times cause a crash
in libspandsp. This would occur when, during fax tone detection, a ulaw/alaw
frame would be passed to modem_connect_tones_rx. That particular routine
expects the data to be in slin format. This patch looks at the frame type and,
if the data is ulaw/alaw, converts the format to slin before passing it to
modem_connect_tones_rx.

Review: https://reviewboard.asterisk.org/r/3296

(closes issue ASTERISK-20149)
Reported by: Alexandr Gordeev
Tested by: Michal Rybarik
patches:
  spandsp_g711decode.diff uploaded by Michal Rybarik (license 6578)
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Merged revisions 409990 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 409991 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@409996 f38db490-d61c-443f-a65b-d21fe96a405b