base: Use a simple tricky to remove a temporary variable in MD5DigestToBase16.

base/md5.cc

Index: base/md5.cc
diff --git a/base/md5.cc b/base/md5.cc
index 737c7c47b0fd7d7b4faa561404634d201f03649e..064e28acf102b4dd7ec82e5a71210dea35d344be 100644
--- a/base/md5.cc
+++ b/base/md5.cc
@@ -276,11 +276,10 @@ std::string MD5DigestToBase16(const MD5Digest& digest) {
   std::string ret;
   ret.resize(32);
 
-  int j = 0;
-  for (int i = 0; i < 16; i++) {
+  for (int i = 0, j = 0; i < 16; i++, j += 2) {
     int a = digest.a[i];
-    ret[j++] = zEncode[(a >> 4) & 0xf];
-    ret[j++] = zEncode[a & 0xf];
+    ret[j] = zEncode[(a >> 4) & 0xf];
+    ret[j + 1] = zEncode[a & 0xf];
   }
   return ret;
 }