checkasm: vp8.mc: initialize the full src buffer after ec325742

Fixes "Use of uninitialised value" valgrind warnings in checkasm.

tests/checkasm/vp8dsp.c

@@ -247,7 +247,7 @@ static void check_luma_dc_wht(void)
 }
 
 #define SRC_BUF_STRIDE 32
-#define SRC_BUF_SIZE ((size + 5) * SRC_BUF_STRIDE)
+#define SRC_BUF_SIZE (((size << (size < 16)) + 5) * SRC_BUF_STRIDE)
 // The mc subpixel interpolation filter needs the 2 previous pixels in either
 // direction, the +1 is to make sure the actual load addresses always are
 // unaligned.