app_dial: Fix infinite loop when sending digits.

app_dial: Fix infinite loop when sending digits.

If the called party hangs up while digits are being
sent, -1 is returned to indicate so, but app_dial
was not checking the return value, resulting in
the hangup being lost and looping forever until
the caller manually hangs up the channel. We now
abort if digit sending fails.

ASTERISK-29428 #close

Resolves: #281