Avoid calling memcpy with n == 0

Some callers of these functions ask to copy 0 items from a NULL source,
which would be valid if they were copied in a loop (because NULL would
never be dereferenced), but is declared to be undefined behaviour for
Standard C memcpy. Guard the call to memcpy so that we only call it
if we have more than 0 items, and therefore should have a non-NULL
source pointer.

Detected by running a subset of the test suite with
-Dsanitize=address,undefined on x86_64.

Signed-off-by: Simon McVittie <smcv@debian.org>

gdk/gdkdmabufformats.c

@@ -207,6 +207,7 @@ gdk_dmabuf_formats_new (GdkDmabufFormat *formats,
   self->n_formats = n_formats;
   self->formats = g_new (GdkDmabufFormat, n_formats);
 
+  if (n_formats != 0)
     memcpy (self->formats, formats, n_formats * sizeof (GdkDmabufFormat));
 
   return self;

gtk/gtksymbolicpaintable.c

@@ -110,6 +110,7 @@ gtk_symbolic_paintable_snapshot_symbolic (GtkSymbolicPaintable   *paintable,
         [GTK_SYMBOLIC_COLOR_SUCCESS] = { 0.3046921492332342,0.6015716792553597, 0.023437857633325704, 1.0 }
       };
 
+      if (n_colors != 0)
         memcpy (real_colors, colors, sizeof (GdkRGBA) * n_colors);
 
       iface->snapshot_symbolic (paintable, snapshot, width, height, real_colors, 4);