audio: removed conditional from simple resampler's inner loop.

We never seem to overflow the source buffer now; this might have been a
leftover from a bug that was covered by Vitaly's fixes?

Removing this conditional makes the resampler 10-20% faster. Left an
assert in there for debug builds, in case this still happens.

src/audio/SDL_audiocvt.c

@@ -211,7 +211,8 @@ SDL_ResampleAudioSimple(const int chans, const double rate_incr,
 
     while (dst < target) {
         const int pos = ((int)idx) * chans;
-        const float *src = &inbuf[(pos >= finalpos) ? finalpos : pos];
+        const float *src = &inbuf[pos];
+        SDL_assert(pos <= finalpos);
         for (i = 0; i < chans; i++) {
             const float val = *(src++);
             *(dst++) = (val + last_sample[i]) * 0.5f;