Fix bad usage of strncat introduces in 1a94c70. Pointed out by @rouault…

… in PR json-c#331.

json_object.c

@@ -774,12 +774,11 @@ static int json_object_double_to_json_string_format(struct json_object* jso,
 		if (!format)
 			format = std_format;
 		size = snprintf(buf, sizeof(buf), format, jso->o.c_double);
-		if (modf(jso->o.c_double, &dummy) == 0)
+		if (modf(jso->o.c_double, &dummy) == 0  && size >= 0 && size < (int)sizeof(buf) - 2)
 		{
 			// Ensure it looks like a float, even if snprintf didn't.
-			strncat(buf, ".0", sizeof(buf) - 1);
-			if (size >= 0)
-				size += 2; // yes, even if strncat ran out of room
+			strcat(buf, ".0");
+			size += 2;
 		}
 	}
 	// although unlikely, snprintf can fail