add a paranoid assert

src/opts/SkBlitRow_opts_SSE4.cpp

 #include "SkBlitRow_opts_SSE4.h"
 
 // Some compilers can't compile SSSE3 or SSE4 intrinsics.  We give them stub methods.
 // The stubs should never be called, so we make them crash just to confirm that.
 #if SK_CPU_SSE_LEVEL < SK_CPU_SSE_LEVEL_SSE41
 void S32A_Opaque_BlitRow32_SSE4(SkPMColor* SK_RESTRICT, const SkPMColor* SK_RESTRICT, int, U8CPU) {
     sk_throw();
 }
 
 #else
@@ skipping 37 matching lines @@
             continue;
         }
         // The general slow case: do the blend for all 16 pixels.
         _mm_storeu_si128(dst4+i+0, SkPMSrcOver_SSE2(s0, _mm_loadu_si128(dst4+i+0)));
         _mm_storeu_si128(dst4+i+1, SkPMSrcOver_SSE2(s1, _mm_loadu_si128(dst4+i+1)));
         _mm_storeu_si128(dst4+i+2, SkPMSrcOver_SSE2(s2, _mm_loadu_si128(dst4+i+2)));
         _mm_storeu_si128(dst4+i+3, SkPMSrcOver_SSE2(s3, _mm_loadu_si128(dst4+i+3)));
     }
 
     // Wrap up the last <= 15 pixels.
+    SkASSERT(count - (count16*16) <= 15);
     for (int i = count16*16; i < count; i++) {
         // This check is not really necessarily, but it prevents pointless autovectorization.
         if (src[i] & 0xFF000000) {
             dst[i] = SkPMSrcOver(src[i], dst[i]);
         }
     }
 }
 
 #endif