scsi: mptfusion: Fix null pointer dereferences in mptscsih_remove()

The mptscsih_remove() function triggers a kernel oops if the Scsi_Host
pointer (ioc->sh) is NULL, as can be seen in this syslog:

 ioc0: LSI53C1030 B2: Capabilities={Initiator,Target}
 Begin: Waiting for root file system ...
 scsi host2: error handler thread failed to spawn, error = -4
 mptspi: ioc0: WARNING - Unable to register controller with SCSI subsystem
 Backtrace:
  [<000000001045b7cc>] mptspi_probe+0x248/0x3d0 [mptspi]
  [<0000000040946470>] pci_device_probe+0x1ac/0x2d8
  [<0000000040add668>] really_probe+0x1bc/0x988
  [<0000000040ade704>] driver_probe_device+0x160/0x218
  [<0000000040adee24>] device_driver_attach+0x160/0x188
  [<0000000040adef90>] __driver_attach+0x144/0x320
  [<0000000040ad7c78>] bus_for_each_dev+0xd4/0x158
  [<0000000040adc138>] driver_attach+0x4c/0x80
  [<0000000040adb3ec>] bus_add_driver+0x3e0/0x498
  [<0000000040ae0130>] driver_register+0xf4/0x298
  [<00000000409450c4>] __pci_register_driver+0x78/0xa8
  [<000000000007d248>] mptspi_init+0x18c/0x1c4 [mptspi]

This patch adds the necessary NULL-pointer checks.  Successfully tested on
a HP C8000 parisc workstation with buggy SCSI drives.

Link: https://lore.kernel.org/r/20201022090005.GA9000@ls3530.fritz.box
Cc: <stable@vger.kernel.org>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/message/fusion/mptscsih.c

@@ -1176,8 +1176,10 @@ mptscsih_remove(struct pci_dev *pdev)
 	MPT_SCSI_HOST		*hd;
 	int sz1;
 
-	if((hd = shost_priv(host)) == NULL)
-		return;
+	if (host == NULL)
+		hd = NULL;
+	else
+		hd = shost_priv(host);
 
 	mptscsih_shutdown(pdev);
 
@@ -1193,14 +1195,15 @@ mptscsih_remove(struct pci_dev *pdev)
 	    "Free'd ScsiLookup (%d) memory\n",
 	    ioc->name, sz1));
 
+	if (hd)
 		kfree(hd->info_kbuf);
 
 	/* NULL the Scsi_Host pointer
 	 */
 	ioc->sh = NULL;
 
+	if (host)
 		scsi_host_put(host);
-
 	mpt_detach(pdev);
 
 }