bignum.c: rb_int_parse_cstr handle `0` strings

	[Bug #19390]

	We shouldn't check the string length when skipping zeros, as the
	string might only contains zero characters, resulting in an empty string.
	---
	 bignum.c               | 1 -
	 test/ruby/test_time.rb | 4 ++++
	 2 files changed, 4 insertions(+), 1 deletion(-)