String optimisations

src/ProtocolBuffers/CodedInputStream.cs

@@ -219,17 +219,20 @@ namespace Google.ProtocolBuffers {
     /// </summary>
     public String ReadString() {
       int size = (int) ReadRawVarint32();
-      if (size < bufferSize - bufferPos && size > 0) {
+      // No need to read any data for an empty string.
+      if (size == 0) {
+        return "";
+      }
+      if (size <= bufferSize - bufferPos) {
         // Fast path:  We already have the bytes in a contiguous buffer, so
         //   just copy directly from it.
         String result = Encoding.UTF8.GetString(buffer, bufferPos, size);
         bufferPos += size;
         return result;
-      } else {
+      }
       // Slow path:  Build a byte array first then copy it.
       return Encoding.UTF8.GetString(ReadRawBytes(size));
     }
-    }
 
     /// <summary>
     /// Reads a group field value from the stream.