Make to_str consistent between backends for negative pointer sizes

[ankane]

Also added more tests for the current behavior.

require "fiddle"

ptr = Fiddle::Pointer["hello\0world"]
ptr.size = 0

p (ptr + 1).size
p (ptr + 1).to_s(5)
p (ptr + 1).to_s
p (ptr + 1).to_str(5)
p (ptr + 1).to_str

Output

-1
"ello\x00"
"ello"
"ello\x00"
repro.rb:10:in 'Fiddle::Pointer#to_str': negative string size (or size too big) (ArgumentError)

[kou]

Thanks.

[ankane]

Thanks