The memcpy and related functions expects a valid pointer, even if the
size is zero. Most libc implementations will handle a NULL pointer just
fine, but that's not guaranteed.
Simply skip the call when there is nothing to copy.
In the error handling code, the dc_buffer_free() function can be called
with an unitialized "buffer" variable as parameter. Fixed by adding an
extra label.
Reported-By: Linus Torvalds <torvalds@linux-foundation.org>