ac3_fixed: fix computation of spx_noise_blend

It was set to 1 instead of sqrt(3)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ac3dec.c

@@ -939,7 +939,7 @@ static int decode_audio_block(AC3DecodeContext *s, int blk)
                             nblend = 0;
                             sblend = 0x800000;
                         } else if (nratio > 0x7fffff) {
-                            nblend = 0x800000;
+                            nblend = 14529495; // sqrt(3) in FP.23
                             sblend = 0;
                         } else {
                             nblend = fixed_sqrt(nratio, 23);