Ruby

Fix floor when ndigits is large

[Bug #20654]

This commit fixes Integer#floor and Float#floor when the number is
negative and ndigits is large such that 10**ndigits is a bignum.

Previously, it would return 0 in such cases. However, this would cause
unexpected behaviour such as:

puts -1.floor(-5) # => -100000 puts -1.floor(-10) # => -10000000000 puts -1.floor(-20) # => 0 

This commit changes the last result so that it will return
-100000000000000000000.