[1611376]by reserving the max string-size when escaped chars exist, we should save some runtime.

src/lib_json/json_writer.cpp

@@ -70,7 +70,10 @@ std::string valueToQuotedString( const char *value )
    // We have to walk value and escape any special characters.
    // Appending to std::string is not efficient, but this should be rare.
    // (Note: forward slashes are *not* rare, but I am not escaping them.)
-   std::string result("\"");
+   unsigned maxsize = strlen(value)*2 + 3; // allescaped+quotes+NULL
+   std::string result;
+   result.reserve(maxsize); // to avoid lots of mallocs
+   result += "\"";
    for (const char* c=value; *c != 0; ++c){
       switch(*c){
          case '\"':
@@ -102,7 +105,8 @@ std::string valueToQuotedString( const char *value )
 	    result += *c;
       }
    }
-   return result + "\"";
+   result += "\"";
+   return result;
 }
 
 // Class Writer