Prevent overflow in rl_set_screen_size

GDB calls rl_set_screen_size in readline with the current screen size,
measured in rows and columns.  To represent "infinite" sizes, GDB
passes in INT_MAX; however, since rl_set_screen_size internally
multiplies the number of rows and columns, this causes a signed
integer overflow.  To prevent this we can instead pass in the
approximate square root of INT_MAX (which is still reasonably large),
so that even when the number of rows and columns is "infinite" we
don't overflow.

gdb/ChangeLog:
2019-02-27  Saagar Jha  <saagar@saagarjha.com>
	    Pedro Alves  <palves@redhat.com>

	* utils.c (set_screen_size): Reduce "infinite" rows and columns
	before calling rl_set_screen_size.