sim_disk.c: only free filebuf if it was allocated here too.

PDP11/pdp11_rq.c re-uses ->filebuf as a sector buffer (under the #defined
name rqxb) and allocates it as such. If an RQ disk is detached and
another attached, this buffer would be lost and the pointer reset to
NULL. sim_disk.c would only allocate the buffer if UNIT_BUFABLE is set,
which means to buffer the whole disk. Since this rightly isn't set on RQ
disks, the pointer would remain NULL and segfaults would ensue.  See
open-simh issue 274.
add comment about safe re-use of filebuf

PDP11/pdp11_rq.c

@@ -172,6 +172,7 @@ extern int32 MMR2;
 #define unit_plug       u4                              /* drive unit plug value */
 #define io_status       u5                              /* io status from callback */
 #define io_complete     u6                              /* io completion flag */
+/* we can re-use filebuf because we don't set UNIT_BUFABLE in flags */
 #define rqxb            filebuf                         /* xfer buffer */
 #define RQ_RMV(u)       ((drv_tab[GET_DTYPE (u->flags)].flgs & RQDF_RMV)? \
                         UF_RMV: 0)