Fixes Clang static analyzer warning:

mate-thumbnail-pixbuf-utils.c:160:17: warning: Division by zero
                                *dest++ = r / n_pixels;
                                          ~~^~~~~~~~~~

Fixes Clang static analyzer warning:

mate-bg.c:2764:2: warning: Potential leak of memory pointed to by 'size'
        g_queue_push_tail (parser->stack, g_strdup (name));
        ^~~~~~~~~~~~~~~~~

Fixes Clang static analyzer warnings:

mate-rr-config.c:549:3: warning: Call to function 'strcpy' is insecure as it does not provide bounding of the memory buffer. Replace unbounded copy functions with analogous functions that support length arguments such as 'strlcpy'. CWE-119
                strcpy (output->priv->vendor, "???");
                ^~~~~~

mate-desktop-item.c:2118:2: warning: Call to function 'strcpy' is insecure as it does not provide bounding of the memory buffer. Replace unbounded copy functions with analogous functions that support length arguments such as 'strlcpy'. CWE-119
        strcpy (the_exec, exec);
        ^~~~~~

Fixes Clang static analyzer warning:

test-ditem.c:94:2: warning: Call to function 'strcat' is insecure as it does not provide bounding of the memory buffer. Replace unbounded copy functions with analogous functions that support length arguments such as 'strlcat'. CWE-119
        strcat (path, "/foo.desktop");
        ^~~~~~

Changed MateBgCrossfade, MateColorbutton and MateDesktopThumbnail