Optimize IsHex() string helper function

This looks like a silly optimization, but this function gets called for
transaction processing, so returning early when there's no point in even
starting to iterate through a string is just smart coding.

src/utilstrencodings.cpp

@@ -58,12 +58,16 @@ signed char HexDigit(char c)
 
 bool IsHex(const string& str)
 {
+    if ((str.size() <= 0) || (str.size()%2 != 0))
+        return false;
+
     for(std::string::const_iterator it(str.begin()); it != str.end(); ++it)
     {
         if (HexDigit(*it) < 0)
             return false;
     }
-    return (str.size() > 0) && (str.size()%2 == 0);
+
+    return true;
 }
 
 vector<unsigned char> ParseHex(const char* psz)