Handle empty input in String's FromBytesWithInput implementation

[oherrala]

This fixes "attempt to subtract with overflow" error when the input buffer is empty. In this case buffer.position() + input - 1 is 0 + 0 - 1 leading to unsigned integer overflow.

In release builds integer overflows are not checked which could lead into out of bounds read of the buffer.

While here validate UTF-8 string before allocating memory for it.

Error from included test before fix is applied:

---- test::test_nl_string_empty stdout ----

thread 'test::test_nl_string_empty' panicked at src/lib.rs:437:46:
attempt to subtract with overflow
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

neli/src/lib.rs

Lines 435 to 439 in 7c6a1f9

	let s = String::from_utf8(
	buffer.get_ref().as_ref()
	[buffer.position() as usize..buffer.position() as usize + input - 1]
	.to_vec(),
	)?;

[jbaublitz]

@oherrala Are you going to resubmit this?

[oherrala]

@jbaublitz I was too hasty and misunderstood some of the code. I opened #307 with maybe more understanding of the code on my part. Sorry about the noise.