[prefix] Avoid using base memory for temporary decompression area

In the unlikely (but observable) event that INT 15,88 returns less
memory above 1MB than is required for the temporary decompression
area, ignore it and use the 1MB point anyway.

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

src/arch/i386/prefix/libprefix.S

@@ -753,21 +753,25 @@ payload_death_message:
 	lcall	*init_libkir_vector
 #else
 	/* Find a suitable decompression temporary area, if none specified */
+	pushl	%eax
 	testl	%ebp, %ebp
 	jnz	1f
 	/* Use INT 15,88 to find the highest available address via INT
 	 * 15,88.  This limits us to around 64MB, which should avoid
 	 * all of the POST-time memory map failure modes.
 	 */
-	pushl	%eax
 	movb	$0x88, %ah
 	int	$0x15
 	movw	%ax, %bp
 	addl	$0x400, %ebp
 	subl	$_textdata_memsz_kb, %ebp
 	shll	$10, %ebp
-	popl	%eax
-1:
+	/* Sanity check: if we have ended up below 1MB, use 1MB */
+	cmpl	$0x100000, %ebp
+	jae	1f
+	movl	$0x100000, %ebp
+1:	popl	%eax
+
 	/* Install .text and .data to temporary area in high memory,
 	 * prior to reading the E820 memory map and relocating
 	 * properly.