chan_vpb: Fix a gcc 7 out-of-bounds complaint

chan_vpb was trying to use sizeof(*p->play_dtmf), where
p->play_dtmf is defined as char[16], to get the length of the array
but since p->play_dtmf is an actual array, sizeof(*p->play_dtmf)
returns the size of the first array element, which is 1.  gcc7
validly complains because the context in which it's used could
cause an out-of-bounds condition.

Change-Id: If9c4bfdb6b02fa72d39e0c09bf88900663c000ba