when calling this function to append to a dynamic string and the buffer was not

when calling this function to append to a dynamic string and the buffer was not
large enough and had to be reallocated, cut off the partially appended data.
Otherwise, the function will get called over and over again appending to the
end every time and never thinking it has enough room.

Thanks to jmls for access to his machine for debugging!

(issue #7691)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@41944 65c4cc65-6c06-0410-ace0-fbb531ad65f3