vt: Fix a read-past-array in vc_t416_color().

This makes it show up on UBSAN:
perl -e 'for (0..15) {my @x=("0")x$_;push @x,qw(38 2 64 128 192 4);printf
"\e[%smAfter %d zeroes.\e[0m\n", join(";",@x[0..($_+5<15?$_+5:15)]), $_}'

Seems harmless: if you can programmatically read attributes of a vt
character (/dev/vcsa*), multiple probes can obtain parts of vt_mode then
lowest byte (5th on 64-bit big-endian) of a pointer.
Signed-off-by: NAdam Borowski <kilobyte@angband.pl>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vt.c

@@ -1316,7 +1316,7 @@ static int vc_t416_color(struct vc_data *vc, int i,
 		/* 256 colours -- ubiquitous */
 		i++;
 		rgb_from_256(vc->vc_par[i], &c);
-	} else if (vc->vc_par[i] == 2 && i <= vc->vc_npar + 3) {
+	} else if (vc->vc_par[i] == 2 && i + 3 <= vc->vc_npar) {
 		/* 24 bit -- extremely rare */
 		c.r = vc->vc_par[i + 1];
 		c.g = vc->vc_par[i + 2];