Avoid spurious gcc warning in debugger.c

Avoid spurious gcc warning in debugger.c

Without this patch, gcc 8.2.0-7 complains:

debugger.c: In function ‘debugger_is_active’:
debugger.c:40:24: warning: passing argument 2 to restrict-qualified parameter aliases with argument 1 [-Wrestrict]
     if (readlink (buf, buf, sizeof (buf)) != -1 &&
                   ~~~  ^~~

This is pretty silly, but it seems simplest to just avoid passing the
same buffer to readlink as both pathname and buf.