Fix memory corruption when decoding Read Response PDU

Fix memory corruption when decoding Read Response PDU

A bogus (or hostile) Proximity Reporter device may send a TX Power value
bigger than the buffer used. Therefore, create a temporary buffer with
the maximum size, and check for the length before using the value.

Note that all other current users of the dec_read_resp() already do
this. Another option would be to change dec_read_resp() to accept a
buffer length, but this would break external code, so it is avoided for
now.