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

Fixes the following memory leaks reported by valgrind:

    32 bytes in 1 blocks are definitely lost in loss record 15 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 0x14B16F: detect_driver (driver.c:20)
       by 0x119949: session_init (compton.c:1879)
       by 0x11AEF0: main (compton.c:2285)

    32 bytes in 1 blocks are definitely lost in loss record 16 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 0x14B16F: detect_driver (driver.c:20)
       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)

Also, free(randr_version) in the error path as well.

src/backend/driver.c

@@ -22,6 +22,7 @@ enum driver detect_driver(xcb_connection_t *c, backend_t *backend_data, xcb_wind
 		    c, xcb_randr_get_providers(c, window), NULL);
 		if (r == NULL) {
 			log_warn("Failed to get RANDR providers");
+			free(randr_version);
 			return 0;
 		}
 
@@ -57,6 +58,7 @@ enum driver detect_driver(xcb_connection_t *c, backend_t *backend_data, xcb_wind
 		}
 		free(r);
 	}
+	free(randr_version);
 
 	// If the backend supports driver detection, use that as well
 	if (backend_data && backend_data->ops->detect_driver) {