Avoid undefined behavior in parse_number

Avoid undefined behavior in parse_number

-fsanitize=undefined pointed out that c-exp.y relied on undefined
behavior here:

      if (c != 'l' && c != 'u')
	n *= base;

...when a large hex constant "just fit" into a LONGEST, causing the
high bit to be set.

This fixes the problem by having the function work in an unsigned
type.

gdb/ChangeLog
2018-10-03  Tom Tromey  <tom@tromey.com>

	* c-exp.y (parse_number): Work in unsigned.  Remove casts.