util/nvmutil rhex: check against sizeof(rnum)

checking against -1 is incorrect, because we specifically want
to ensure that it always read the number of bytes defined by
the size of rnum.

this still covers the case where the return value is -1, and
therefore makes the error handling much stricter.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/nvmutil.c

@@ -368,7 +368,7 @@ rhex(void)
 
 	if (!n) {
 		n = sizeof(rnum) - 1;
-		err_if(read(rfd, (uint8_t *) &rnum, n + 1) == -1);
+		err_if(read(rfd, (uint8_t *) &rnum, n + 1) != sizeof(rnum));
 	}
 
 	return rnum[n--] & 0xf;