[XERCESC-2180] Remove assertion when a surrogate pair is split by the boundary of an input buffer (transcoders try to avoid this, but UTF-16 transcoder doesn't have this check in place). The reader now pulls in more data on demand.
xerces/c/trunk/src/xercesc/internal/XMLReader.cpp
xerces/c/trunk/tests/src/XSTSHarness/regression/XERCESC-2180/