tests: fix bdrv-drain leak

Spotted by ASAN:

=================================================================
==5378==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 65536 byte(s) in 1 object(s) allocated from:
    #0 0x7f788f83bc48 in malloc (/lib64/libasan.so.5+0xeec48)
    #1 0x7f788c9923c5 in g_malloc (/lib64/libglib-2.0.so.0+0x523c5)
    #2 0x5622a1fe37bc in coroutine_trampoline /home/elmarco/src/qq/util/coroutine-ucontext.c:116
    #3 0x7f788a15d75f in __correctly_grouped_prefixwc (/lib64/libc.so.6+0x4c75f)

(Broken in commit 4c8158e3.)
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20180809114417.28718-3-marcandre.lureau@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/test-bdrv-drain.c

@@ -948,6 +948,7 @@ static void coroutine_fn test_co_delete_by_drain(void *opaque)
     }
 
     dbdd->done = true;
+    g_free(buffer);
 }
 
 /**