avformat/mpegts: Reset local nb_prg on add_program() failure

add_program() will deallocate the whole array on failure so
we must clear nb_prgs

Fixes: null pointer dereference
Fixes: crash-35a3b39ddcc5babeeb005b7399a3a1217c8781bc

Found-by: Catena cyber
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit cb9752d897)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mpegts.c

@@ -2598,7 +2598,8 @@ static void pat_cb(MpegTSFilter *filter, const uint8_t *section, int section_len
                     FFSWAP(struct Program, ts->prg[nb_prg], ts->prg[prg_idx]);
                 if (prg_idx >= nb_prg)
                     nb_prg++;
-            }
+            } else
+                nb_prg = 0;
         }
     }
     ts->nb_prg = nb_prg;