CLEANUP: ncbmbuf: improve handling of memory allocation errors in unit tests

CLEANUP: ncbmbuf: improve handling of memory allocation errors in unit tests

Found via cppcheck  --force --enable=all --output-file=haproxy.log :

src/ncbmbuf.c:192:9: warning: If memory allocation fails, then there is a possible null pointer dereference: area [nullPointerOutOfMemory]
src/ncbmbuf.c:373:9: warning: If memory allocation fails, then there is a possible null pointer dereference: data [nullPointerOutOfMemory]
src/ncbmbuf.c:546:9: warning: If memory allocation fails, then there is a possible null pointer dereference: data [nullPointerOutOfMemory]