disk/lvm: Bail on missing PV list

There's an if block for the presence of "physical_volumes {", but if
that block is absent, then p remains NULL and a NULL-deref will result
when looking for logical volumes.

It doesn't seem like LVM makes sense without physical volumes, so error
out rather than crashing.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/disk/lvm.c

@@ -409,6 +409,8 @@ grub_lvm_detect (grub_disk_t disk,
 	      goto fail4;
 	    }
 	}
+      else
+        goto fail4;
 
       p = grub_strstr (p, "logical_volumes {");
       if (p)