avcodec/cbs_h265: move the payload_extension_present check into its own function

Will be reused in the following patch.
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/cbs_h2645.c

@@ -233,6 +233,16 @@ static int cbs_write_se_golomb(CodedBitstreamContext *ctx, PutBitContext *pbc,
     return 0;
 }
 
+// payload_extension_present() - true if we are before the last 1-bit
+// in the payload structure, which must be in the last byte.
+static int cbs_h265_payload_extension_present(GetBitContext *gbc, uint32_t payload_size,
+                                              int cur_pos)
+{
+    int bits_left = payload_size * 8 - cur_pos;
+    return (bits_left > 0 &&
+            (bits_left > 7 || show_bits(gbc, bits_left) & MAX_UINT_BITS(bits_left - 1)));
+}
+
 #define HEADER(name) do { \
         ff_cbs_trace_header(ctx, name); \
     } while (0)

libavcodec/cbs_h265_syntax_template.c

@@ -1572,7 +1572,7 @@ static int FUNC(sei_buffering_period)(CodedBitstreamContext *ctx, RWContext *rw,
     int err, i, length;
 
 #ifdef READ
-    int start_pos, end_pos, bits_left;
+    int start_pos, end_pos;
     start_pos = get_bits_count(rw);
 #endif
 
@@ -1651,12 +1651,9 @@ static int FUNC(sei_buffering_period)(CodedBitstreamContext *ctx, RWContext *rw,
     }
 
 #ifdef READ
-    // payload_extension_present() - true if we are before the last 1-bit
-    // in the payload structure, which must be in the last byte.
     end_pos = get_bits_count(rw);
-    bits_left = *payload_size * 8 - (end_pos - start_pos);
-    if (bits_left > 0 &&
-        (bits_left > 7 || ff_ctz(show_bits(rw, bits_left)) < bits_left - 1))
+    if (cbs_h265_payload_extension_present(rw, *payload_size,
+                                           end_pos - start_pos))
         flag(use_alt_cpb_params_flag);
     else
         infer(use_alt_cpb_params_flag, 0);