sws/x86: add av_assert() to ensure 16bit code is used with 16bit data

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/x86/swscale_mmx.c

@@ -396,6 +396,7 @@ void ff_sws_init_swScale_mmx(SwsContext *c)
         hscalefn = c->dstBpc <= 14 ? ff_hscale14to15_ ## filtersize ## _ ## opt2 : \
                                      ff_hscale14to19_ ## filtersize ## _ ## opt1; \
     } else { /* c->srcBpc == 16 */ \
+        av_assert0(c->srcBpc == 16);\
         hscalefn = c->dstBpc <= 14 ? ff_hscale16to15_ ## filtersize ## _ ## opt2 : \
                                      ff_hscale16to19_ ## filtersize ## _ ## opt1; \
     } \