Fixed off-by-one error in SDL_ConvertStereo().

Fixes Bugzilla #561.

--HG--
extra : rebase_source : a21159567a0a8a9485c46d4c67e57224948952a0

src/audio/SDL_audiocvt.c

@@ -293,10 +293,9 @@ SDL_ConvertStereo(SDL_AudioCVT * cvt, SDL_AudioFormat format)
         const type *src = (const type *) (cvt->buf + cvt->len_cvt); \
         type *dst = (type *) (cvt->buf + cvt->len_cvt * 2); \
         for (i = cvt->len_cvt / sizeof(type); i; --i) { \
-            const type val = *src; \
             src -= 1; \
             dst -= 2; \
-            dst[0] = dst[1] = val; \
+            dst[0] = dst[1] = *src; \
         } \
     }