avfilter/unsharp: fix uninitialized pointer read

Fixes CID 1396855

libavfilter/unsharp_opencl.c

@@ -43,7 +43,7 @@ static int compute_mask(int step, uint32_t *mask)
 {
     int i, z, ret = 0;
     int counter_size = sizeof(uint32_t) * (2 * step + 1);
-    uint32_t *temp1_counter, *temp2_counter, **counter;
+    uint32_t *temp1_counter, *temp2_counter, **counter = NULL;
     temp1_counter = av_mallocz(counter_size);
     if (!temp1_counter) {
         ret = AVERROR(ENOMEM);
@@ -80,7 +80,7 @@ static int compute_mask(int step, uint32_t *mask)
 end:
     av_freep(&temp1_counter);
     av_freep(&temp2_counter);
-    for (i = 0; i < 2 * step + 1; i++) {
+    for (i = 0; counter && i < 2 * step + 1; i++) {
         av_freep(&counter[i]);
     }
     av_freep(&counter);