Fix ASAN error for wbmp swizzles

BUG=skia:

Review URL: https://codereview.chromium.org/1269413006

src/codec/SkSwizzler.cpp

diff --git a/src/codec/SkSwizzler.cpp b/src/codec/SkSwizzler.cpp
index f8e89c7..4da862f 100644
--- a/src/codec/SkSwizzler.cpp
+++ b/src/codec/SkSwizzler.cpp
@@ -43,10 +43,12 @@
 
     // Finish the remaining bits
     width &= 7;
-    U8CPU currByte = src[i];
-    for (int j = 0; j < width; j++) {
-        dst[j] = ((currByte >> 7) & 1) ? GRAYSCALE_WHITE : GRAYSCALE_BLACK;
-        currByte <<= 1;
+    if (width > 0) {
+        U8CPU currByte = src[i];
+        for (int j = 0; j < width; j++) {
+            dst[j] = ((currByte >> 7) & 1) ? GRAYSCALE_WHITE : GRAYSCALE_BLACK;
+            currByte <<= 1;
+        }
     }
     return SkSwizzler::kOpaque_ResultAlpha;
 }
@@ -72,10 +74,12 @@
 
     // Finish the remaining bits
     width &= 7;
-    U8CPU currByte = src[i];
-    for (int j = 0; j < width; j++) {
-        dst[j] = ((currByte >> 7) & 1);
-        currByte <<= 1;
+    if (width > 0) {
+        U8CPU currByte = src[i];
+        for (int j = 0; j < width; j++) {
+            dst[j] = ((currByte >> 7) & 1);
+            currByte <<= 1;
+        }
     }
     return SkSwizzler::kOpaque_ResultAlpha;
 }
@@ -98,10 +102,12 @@
 
     // Finish the remaining bits
     width &= 7;
-    U8CPU currByte = src[i];
-    for (int j = 0; j < width; j++) {
-        dst[j] = ((currByte >> 7) & 1) ? SK_ColorWHITE : SK_ColorBLACK;
-        currByte <<= 1;
+    if (width > 0) {
+        U8CPU currByte = src[i];
+        for (int j = 0; j < width; j++) {
+            dst[j] = ((currByte >> 7) & 1) ? SK_ColorWHITE : SK_ColorBLACK;
+            currByte <<= 1;
+        }
     }
     return SkSwizzler::kOpaque_ResultAlpha;
 }