rlm_rest bugfix

When web server replies to a REST request with a x-www-form-urlencoded body, if more than one attribute is provided then rlm_rest makes freeradius crash.

The problem appears to be related with the behavior of the "radius_axlat" function. If the pointer to the output buffer provided as parameter isn't NULL, the function uses the current value instead of providing a new buffer; and if the buffer pointed to has already been freed, the process segfaults.