vp9/10 decoder_remove: check pbi pointer

fixes crash on error

Change-Id: Ibb1ef5565fb833cdee1a49335473d98f1187ef43

vp10/decoder/decoder.c

@@ -126,6 +126,9 @@ VP10Decoder *vp10_decoder_create(BufferPool *const pool) {
 void vp10_decoder_remove(VP10Decoder *pbi) {
   int i;
 
+  if (!pbi)
+    return;
+
   vpx_get_worker_interface()->end(&pbi->lf_worker);
   vpx_free(pbi->lf_worker.data1);
   vpx_free(pbi->tile_data);

vp9/decoder/vp9_decoder.c

@@ -126,6 +126,9 @@ VP9Decoder *vp9_decoder_create(BufferPool *const pool) {
 void vp9_decoder_remove(VP9Decoder *pbi) {
   int i;
 
+  if (!pbi)
+    return;
+
   vpx_get_worker_interface()->end(&pbi->lf_worker);
   vpx_free(pbi->lf_worker.data1);
   vpx_free(pbi->tile_data);