Use memmove for overlapping copy [llvm address sanitizer]

(raptor_ntriples_term): Use memmove to replace memcpy for an overlapping
copy.

Found via LLVM address sanitizer in LLVM 3.4 (SVN) with options:
  export ASAN_SYMBOLIZER_PATH=/usr/bin/llvm-symbolizer-3.4
  CC='clang -fsanitize=address -O1 -fno-omit-frame-pointer -g' ./configure ...
See https://code.google.com/p/address-sanitizer/wiki/AddressSanitizer
and http://llvm.org/releases/3.3/tools/clang/docs/AddressSanitizer.html