Decrement value in mysql but not negative

Add another condition to update only if the field is greater 0

UPDATE table 
SET field = field - 1
WHERE id = $number
and field > 0

You could prevent the new value to drop below zero by using GREATEST(). If the value drops below zero, zero will always be greater than your calculated value, thus preventing any value below zero to be used.

UPDATE  table
SET     field = GREATEST(0, field - 1)
WHERE   id = $number

And on a side note: Please don't use mysql_* functions any more. They are deprecated and will eventually be removed from PHP. Use PDO or MySQLi instead.