[h323plus] bug in rtp.cxx

[robertp]

line 883 of rtp.cxx is bugged, you're deleting jitter, even if the audio
disable has been used for the build. This causes a build error.

Also, how reliant is h323plus on the forked sections of ptlib ? Its
rather annoying having to build against a customised version. Would it
be at all reasonable to make a motion towards merging the ptlib found in
other sources such as the ubuntu repositories, and the ptlib given on
the h323plus website ?

Thanks in advance.

- R