Fixed the random crash associated with undecodable RASL pictures. (#141)

* * rocDecode/HEVC: Fixed the random crash associated with undecodable RASL pictures.
  - When a CRA picutre occurs, all the pictures in the DPB are emptied. However, the following RASL (Random Access Skipped Leading) picture can still reference these emptied pictures, making them undecodeble.
  - We initialized the reference picture lists with (0xFF), representing invalid picture index. Normally these invalid indices are replaced by valid values during RPS decoding. However, on undecodable RASL pictures, we can not find an existing reference picture in DPB, leaving the invalid index untouched and resulting invalid indexing later on.
  - Now we initialize the reference picture lists with a valid value 0. Also we add index range check on reference buffer at VA-API layer to avoid invalid memory access.

* * rocDecode/HEVC: Fixed a typo in sampel app message.

src/parser/hevc_parser.cpp

@@ -2073,13 +2073,13 @@ void HEVCVideoParser::DecodeRps() {
         /*
         * RPS derivation and picture marking
         */
-        // Init as "no reference picture"
+        // Init to a valid index value to take care of undecodable RASL pictures, whose reference pictures are emptied after a CRA.
         for (i = 0; i < HEVC_MAX_NUM_REF_PICS; i++) {
-            ref_pic_set_st_curr_before_[i] = 0xFF;
-            ref_pic_set_st_curr_after_[i] = 0xFF;
-            ref_pic_set_st_foll_[i] = 0xFF;
-            ref_pic_set_lt_curr_[i] = 0xFF;
-            ref_pic_set_lt_foll_[i] = 0xFF;
+            ref_pic_set_st_curr_before_[i] = 0;
+            ref_pic_set_st_curr_after_[i] = 0;
+            ref_pic_set_st_foll_[i] = 0;
+            ref_pic_set_lt_curr_[i] = 0;
+            ref_pic_set_lt_foll_[i] = 0;
         }
 
         // Mark all in DPB as unused. We will mark them back while we go through the ref lists. The rest will be actually unused.