opus: always sync i_buffer with i_nb_samples (fixes #10135)

Specifically, the crash at the end of playback was happening when end trim was
not being subtracted from i_buffer.

modules/codec/opus.c

@@ -408,10 +408,12 @@ static block_t *DecodePacket( decoder_t *p_dec, ogg_packet *p_oggpacket,
             msg_Err( p_dec, "Error: corrupted stream?" );
         return NULL;
     }
-    if( spp > i_nb_samples )
-    {
+
     p_aout_buffer->i_buffer = (i_nb_samples - i_end_trim) *
                               p_sys->header.channels * sizeof(float);
+
+    if( spp > i_nb_samples )
+    {
         memmove(p_aout_buffer->p_buffer,
             p_aout_buffer->p_buffer
             + (spp - i_nb_samples)*p_sys->header.channels*sizeof(float),