console/vtxprintf.c: cast precision to size_t for string length

If no maximum string length is specified, we're intentionally passing a
value of -1 to get the string length so that it's not limited.  This
makes checking tools unhappy, so actively cast it to size_t before
passing it into strlen to show that it's not an accident.

Addresses coverity issue 1129133 - Argument cannot be negative

Change-Id: I40f8f2101e170a5c96fcd39c217aa414f4316473
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/17479
Tested-by: build bot (Jenkins)
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/console/vtxprintf.c

@@ -228,7 +228,7 @@ repeat:
 			if (!s)
 				s = "<NULL>";
 
-			len = strnlen(s, precision);
+			len = strnlen(s, (size_t)precision);
 
 			if (!(flags & LEFT))
 				while (len < field_width--)