swr: fix "may be used uninitialized in this function" warnings

The uninitialized use implicates 0 channels, which is unlikely
to reach this function but fixed anyway in this commit.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswresample/dither_template.c

@@ -24,7 +24,8 @@ ERROR
 #endif
 
 void RENAME(swri_noise_shaping)(SwrContext *s, AudioData *dsts, const AudioData *srcs, const AudioData *noises, int count){
-    int i, j, pos, ch;
+    int pos = s->dither.ns_pos;
+    int i, j, ch;
     int taps  = s->dither.ns_taps;
     float S   = s->dither.ns_scale;
     float S_1 = s->dither.ns_scale_1;