[umalloc] Unhide umalloc()ed memory region when there are no allocations

At present, we always hide an extra sizeof(struct external_memory), to
account for the header on the lowest allocated block.  This header
ceases to exist when there are no allocated blocks remaining.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/interface/pcbios/memtop_umalloc.c

 
 	/* Collect any free blocks and update hidden memory region */
 	ecollect_free();
-	hide_umalloc ( user_to_phys ( bottom, 0 ),
+	hide_umalloc ( user_to_phys ( bottom, ( ( bottom == top ) ?
+						0 : -sizeof ( extmem ) ) ),
 		       user_to_phys ( top, 0 ) );
 
 	return ( new_size ? new : UNOWHERE );