dump: do not dump non-existent guest memory

It does not really make sense to dump memory that is not there.

Moreover, that fixes a segmentation fault when calling dump-guest-memory
with no filter for a machine with no memory defined.

New behaviour is:

(qemu) dump-guest-memory /dev/null
dump: no guest memory to dump
(qemu) dump-guest-memory /dev/null 0 4096
dump: no guest memory to dump
Signed-off-by: NCornelia Huck <cohuck@redhat.com>
Tested-by: NLaurent Vivier <lvivier@redhat.com>
Reviewed-by: NLaurent Vivier <lvivier@redhat.com>
Reviewed-by: NGreg Kurz <groug@kaod.org>
Reviewed-by: NPeter Xu <peterx@redhat.com>
Message-Id: <20170913142036.2469-4-lvivier@redhat.com>
Signed-off-by: NLaurent Vivier <lvivier@redhat.com>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>

dump.c

@@ -1536,6 +1536,12 @@ static void dump_init(DumpState *s, int fd, bool has_format,
     fprintf(stderr, "DUMP: total memory to dump: %lu\n", s->total_size);
 #endif
 
+    /* it does not make sense to dump non-existent memory */
+    if (!s->total_size) {
+        error_setg(errp, "dump: no guest memory to dump");
+        goto cleanup;
+    }
+
     s->start = get_start_block(s);
     if (s->start == -1) {
         error_setg(errp, QERR_INVALID_PARAMETER, "begin");