cmdutils_opencl: Fix read of uinitialized cl_mem

Fixes CID1396856, CID1396860, CID1396861
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

cmdutils_opencl.c

@@ -133,7 +133,7 @@ static int64_t run_opencl_bench(AVOpenCLExternalEnv *ext_opencl_env)
     int buf_size = width * height * sizeof(char);
     int mask_size = sizeof(uint32_t) * 128;
 
-    cl_mem cl_mask, cl_inbuf, cl_outbuf;
+    cl_mem cl_mask = NULL, cl_inbuf = NULL, cl_outbuf = NULL;
     cl_kernel kernel = NULL;
     cl_program program = NULL;
     size_t local_work_size_2d[2] = {16, 16};