cmdutils: move grow_array() from avconv to cmdutils.

cmdutils.c

@@ -931,4 +931,23 @@ int get_filtered_video_frame(AVFilterContext *ctx, AVFrame *frame,
     return 1;
 }
 
+void *grow_array(void *array, int elem_size, int *size, int new_size)
+{
+    if (new_size >= INT_MAX / elem_size) {
+        av_log(NULL, AV_LOG_ERROR, "Array too big.\n");
+        exit_program(1);
+    }
+    if (*size < new_size) {
+        uint8_t *tmp = av_realloc(array, new_size*elem_size);
+        if (!tmp) {
+            av_log(NULL, AV_LOG_ERROR, "Could not alloc buffer.\n");
+            exit_program(1);
+        }
+        memset(tmp + *size*elem_size, 0, (new_size-*size) * elem_size);
+        *size = new_size;
+        return tmp;
+    }
+    return array;
+}
+
 #endif /* CONFIG_AVFILTER */