lavc/startcode: fix RVV return value on no match

If there are no zero bytes, t2 equals -1. The code cannot simply fall
through to the match case.

libavcodec/riscv/startcode_rvv.S

@@ -37,6 +37,8 @@ func ff_startcode_find_candidate_rvv, zve32x
         bgez     t2, 2f
         add      t0, t0, t1
         bnez     a1, 1b
+
+        mv       t2, zero
 2:
         add      t0, t0, t2
         sub      a0, t0, a0