x86: fix buffer overflow in efi_init()

If the vendor name (from c16) can be longer than 100 bytes (or missing a
terminating null), then the null is written past the end of vendor[].

Found with Parfait, http://research.sun.com/projects/parfait/

Signed-off-by: Roel Kluin <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: H. Peter Anvin <[e-mail]>
Cc: Huang Ying <[e-mail]>

arch/x86/kernel/efi.c

···
 	 */
 	c16 = tmp = early_ioremap(efi.systab->fw_vendor, 2);
 	if (c16) {
-		for (i = 0; i < sizeof(vendor) && *c16; ++i)
+		for (i = 0; i < sizeof(vendor) - 1 && *c16; ++i)
 			vendor[i] = *c16++;
 		vendor[i] = '\0';
 	} else