coap-server.c: avoid fopen on NULL pointer in read_file_mem
coap-server.c: avoid fopen on NULL pointer in read_file_mem

As the behavior for calling fopen() on a NULL pointer is undefined in
ISO C99, a check is added before passing the file name to fopen().

This fixes CID 1495723.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap