lavu/opt: validate range before dereference

This change make error handling simplier.
av_opt_freep_ranges may be called when some ranges are NULL,
for example after memory allocation fail.

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 3aac5fcfa9)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/opt.c

@@ -1599,9 +1599,11 @@ void av_opt_freep_ranges(AVOptionRanges **rangesp)
 
     for (i = 0; i < ranges->nb_ranges; i++) {
         AVOptionRange *range = ranges->range[i];
+        if (range) {
             av_freep(&range->str);
             av_freep(&ranges->range[i]);
         }
+    }
     av_freep(&ranges->range);
     av_freep(rangesp);
 }