Fix static analyzer warnings.

Fix the following warning:
frameworks/native/libs/binder/Value.cpp:187:11: warning: Use of memory
after it is freed [clang-analyzer-cplusplus.NewDelete]

Bug: b/27101951
Test:Warnings are gone.
Change-Id: Iebd91d87dd740a39c690c25d690960f7e43d6dbe
diff --git a/libs/binder/Value.cpp b/libs/binder/Value.cpp
index fd1dfd5..85cd739 100644
--- a/libs/binder/Value.cpp
+++ b/libs/binder/Value.cpp
@@ -182,10 +182,12 @@
 
 Value& Value::operator=(const Value& rhs)
 {
-    delete mContent;
-    mContent = rhs.mContent
-        ? rhs.mContent->clone()
-        : NULL;
+    if (this != &rhs) {
+        delete mContent;
+        mContent = rhs.mContent
+            ? rhs.mContent->clone()
+            : NULL;
+    }
     return *this;
 }