Fix panic when parsing improper scientific notation.

[schungx]

This fixes #697

[paupino]

Thank you for fixing this!

[paupino]

Minor: Stylistically, I'd probably shift this test case to use a Result instead - just to keep "the way to write tests" consistent (e.g. like this:

rust-decimal/tests/decimal_tests.rs

Lines 3243 to 3275 in 46fb4c3

	fn it_can_parse_exact_highly_significant_numbers() {
	use rust_decimal::Error;
	let tests = &[
	(
	"11.111111111111111111111111111",
	Ok("11.111111111111111111111111111".to_string()),
	),
	("11.11111111111111111111111111111", Err(Error::Underflow)),
	("11.1111111111111111111111111115", Err(Error::Underflow)),
	("115.111111111111111111111111111", Err(Error::Underflow)),
	("1115.11111111111111111111111111", Err(Error::Underflow)),
	("11.1111111111111111111111111195", Err(Error::Underflow)),
	("99.9999999999999999999999999995", Err(Error::Underflow)),
	("-11.1111111111111111111111111195", Err(Error::Underflow)),
	("-99.9999999999999999999999999995", Err(Error::Underflow)),
	(
	"3.1415926535897932384626433832",
	Ok("3.1415926535897932384626433832".to_string()),
	),
	("8808257419827262908.5944405087133154018", Err(Error::Underflow)),
	("8097370036018690744.2590371109596744091", Err(Error::Underflow)),
	("8097370036018690744.2590371149596744091", Err(Error::Underflow)),
	("8097370036018690744.2590371159596744091", Err(Error::Underflow)),
	("1.234567890123456789012345678949999", Err(Error::Underflow)),
	(".00000000000000000000000000001", Err(Error::Underflow)),
	(".10000000000000000000000000000", Err(Error::Underflow)),
	];
	for &(value, ref expected) in tests.iter() {
	let actual = Decimal::from_str_exact(value).map(|d| d.to_string());
	assert_eq!(*expected, actual);
	}
	}

)