Fix SIP INFO DTMF handling for non-numeric codes

Fix SIP INFO DTMF handling for non-numeric codes

In ASTERISK-18924, SIP INFO DTMF handlingw as changed to account for both
lowercase alphatbetic DTMF events, as well as uppercase alphabetic DTMF
events.  When this occurred, the comparison of the character buffer containing
the event code was changed such that the buffer was first compared again '0'
and '9' to determine if it was numeric.  Unfortunately, since the first
character in the buffer will typically be '1' in the case of non-numeric
event codes (10-16), this caused those codes to be converted to a DTMF event
of '1'.  This patch fixes that, and cleans up handling of both
application/dtmf-relay and application/dtmf content types.

Review: https://reviewboard.asterisk.org/r/1722/

(closes issue ASTERISK-19290)
Reported by: Ira Emus
Tested by: mjordan

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@354542 65c4cc65-6c06-0410-ace0-fbb531ad65f3