opt: do not crash in av_set_options_string() if opts == NULL

Add missing NULL check, and update documentation accordingly.

libavutil/avutil.h

@@ -41,7 +41,7 @@
 
 #define LIBAVUTIL_VERSION_MAJOR 51
 #define LIBAVUTIL_VERSION_MINOR  9
-#define LIBAVUTIL_VERSION_MICRO  0
+#define LIBAVUTIL_VERSION_MICRO  1
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \
                                                LIBAVUTIL_VERSION_MINOR, \

libavutil/opt.c

@@ -521,6 +521,8 @@ int av_set_options_string(void *ctx, const char *opts,
 {
     int ret, count = 0;
 
+    if (!opts)
+        return 0;
     while (*opts) {
         if ((ret = parse_key_value_pair(ctx, &opts, key_val_sep, pairs_sep)) < 0)
             return ret;

libavutil/opt.h

@@ -169,6 +169,7 @@ void av_opt_set_defaults2(void *s, int mask, int flags);
  * key. ctx must be an AVClass context, storing is done using
  * AVOptions.
  *
+ * @param opts options string to parse, may be NULL
  * @param key_val_sep a 0-terminated list of characters used to
  * separate key from value
  * @param pairs_sep a 0-terminated list of characters used to separate