Fix sccp_read_data when reading segmented packets (for example XMLAlarmMessage). memcpy & buffering is needed to be able to skip the header of the next segment. Previous implementation overwrote msg->data with next data from later segments.

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5033 43c1dc8c-776e-4df2-8ef1-6e829299be21