backend: driver: fix memory leak (r2 not freed)

Fixes the following memory leaks reported by valgrind:

    96 bytes in 1 blocks are definitely lost in loss record 54 of 111
       at 0x483577F: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
       by 0x49DB0DD: ??? (in /usr/lib/x86_64-linux-gnu/libxcb.so.1.1.0)
       by 0x49D8FB4: ??? (in /usr/lib/x86_64-linux-gnu/libxcb.so.1.1.0)
       by 0x49DA63E: ??? (in /usr/lib/x86_64-linux-gnu/libxcb.so.1.1.0)
       by 0x49DA751: xcb_wait_for_reply (in /usr/lib/x86_64-linux-gnu/libxcb.so.1.1.0)
       by 0x14B266: detect_driver (driver.c:34)
       by 0x119949: session_init (compton.c:1879)
       by 0x11AEF0: main (compton.c:2285)

    96 bytes in 1 blocks are definitely lost in loss record 55 of 111
       at 0x483577F: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
       by 0x49DB0DD: ??? (in /usr/lib/x86_64-linux-gnu/libxcb.so.1.1.0)
       by 0x49D8FB4: ??? (in /usr/lib/x86_64-linux-gnu/libxcb.so.1.1.0)
       by 0x49DA63E: ??? (in /usr/lib/x86_64-linux-gnu/libxcb.so.1.1.0)
       by 0x49DA751: xcb_wait_for_reply (in /usr/lib/x86_64-linux-gnu/libxcb.so.1.1.0)
       by 0x14B266: detect_driver (driver.c:34)
       by 0x11795E: redir_start (compton.c:1202)
       by 0x115C76: paint_preprocess (compton.c:629)
       by 0x1182BD: _draw_callback (compton.c:1380)
       by 0x1184D0: draw_callback (compton.c:1426)
       by 0x49BF292: ev_invoke_pending (in /usr/lib/x86_64-linux-gnu/libev.so.4.0.0)
       by 0x49C3344: ev_run (in /usr/lib/x86_64-linux-gnu/libev.so.4.0.0)

src/backend/driver.c

@@ -55,6 +55,7 @@ enum driver detect_driver(xcb_connection_t *c, backend_t *backend_data, xcb_wind
 				ret |= DRIVER_INTEL;
 			}
 			free(name);
+			free(r2);
 		}
 		free(r);
 	}