guard against self-assignment

BUG=skia:
R=mtklein@google.com, bungeman@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/264793014

git-svn-id: http://skia.googlecode.com/svn/trunk@14545 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/core/SkPaint.cpp b/src/core/SkPaint.cpp
index 176992f..0e34be6 100644
--- a/src/core/SkPaint.cpp
+++ b/src/core/SkPaint.cpp
@@ -153,6 +153,10 @@
 }
 
 SkPaint& SkPaint::operator=(const SkPaint& src) {
+    if (this == &src) {
+        return *this;
+    }
+
 #define COPY(field) field = src.field
 #define REF_COPY(field) SkSafeUnref(field); field = SkSafeRef(src.field)