Pad 'Left' when building under ASan

The neon intrinsics are not able to load just the 4 values that are
used. In vpx_dsp/arm/intrapred_neon.c:dc_4x4 it loads 8 values for both
the 'above' and 'left' computations, but only uses the sum of the first
4 values.

BUG=webm:1268

Change-Id: I937113d7e3a21e25bebde3593de0446bf6b0115a

vp8/common/reconintra4x4.c

@@ -15,6 +15,8 @@
 #include "vp8_rtcd.h"
 #include "blockd.h"
 #include "reconintra4x4.h"
+#include "vp8/common/common.h"
+#include "vpx_ports/mem.h"
 
 typedef void (*intra_pred_fn)(uint8_t *dst, ptrdiff_t stride,
                               const uint8_t *above, const uint8_t *left);
@@ -38,8 +40,19 @@ void vp8_intra4x4_predict(unsigned char *above, unsigned char *yleft,
                           int left_stride, B_PREDICTION_MODE b_mode,
                           unsigned char *dst, int dst_stride,
                           unsigned char top_left) {
-  unsigned char Left[4];
   unsigned char Aboveb[12], *Above = Aboveb + 4;
+#if HAVE_NEON
+  // Neon intrinsics are unable to load 32 bits, or 4 8 bit values. Instead, it
+  // over reads but does not use the extra 4 values.
+  unsigned char Left[8];
+#if VPX_WITH_ASAN
+  // Silence an 'uninitialized read' warning. Although uninitialized values are
+  // indeed read, they are not used.
+  vp8_zero_array(Left, 8);
+#endif  // VPX_WITH_ASAN
+#else
+  unsigned char Left[4];
+#endif  // HAVE_NEON
 
   Left[0] = yleft[0];
   Left[1] = yleft[left_stride];

vpx_ports/mem.h

@@ -48,4 +48,14 @@
 #define CONVERT_TO_BYTEPTR(x) ((uint8_t *)(((uintptr_t)(x)) >> 1))
 #endif  // CONFIG_VP9_HIGHBITDEPTH
 
+#if !defined(__has_feature)
+#define __has_feature(x) 0
+#endif  // !defined(__has_feature)
+
+#if __has_feature(address_sanitizer) || __SANITIZE_ADDRESS__
+#define VPX_WITH_ASAN 1
+#else
+#define VPX_WITH_ASAN 0
+#endif  // __has_feature(address_sanitizer) || __SANITIZE_ADDRESS
+
 #endif  // VPX_PORTS_MEM_H_