AST-2018-010: Fix length of buffer needed for SRV and NAPTR results

When dn_expand was being called on SRV and NAPTR results, the
return value was being used to calculate the size of the buffer
needed to store the host names.  Since dn_expand returns the
length of the COMPRESSED name the buffer could be too short
to hold the EXPANDED name.  The expanded name is NULL terminated
so using strlen() is the correct way to determine the length
actually needed for the buffer.

ASTERISK-28127
Reported by: Jan Hoffmann

patches:
  patch.diff submitted by janhoffmann (license 6986)

Change-Id: I4d35d6c431c6c6836cb61d37b1378cc47f0b414d