check if console/channel PTY is null before attempting to open

Console/channel devices have their pty devices assigned when the emulator is
actually started. If time is spent in guest preparation, someone attempts
to open the console/channel, the libvirt crashes in virChrdevLockFilePath().
The patch attempts to fix the crash by adding a check before attempting to
open.
Signed-off-by: NShivaprasad G Bhat <sbhat@linux.vnet.ibm.com>

src/conf/virchrdev.c

@@ -350,6 +350,11 @@ int virChrdevOpen(virChrdevsPtr devs,
     switch (source->type) {
     case VIR_DOMAIN_CHR_TYPE_PTY:
         path = source->data.file.path;
+        if (!path) {
+            virReportError(VIR_ERR_OPERATION_FAILED, "%s",
+                           _("PTY device is not yet assigned"));
+            return -1;
+        }
         break;
     case VIR_DOMAIN_CHR_TYPE_UNIX:
         path = source->data.nix.path;