Fix incorrect string len if an exception's message is invalid UTF-8

[dtolnay]

Previously we'd create a PtrLen with the C++ string's length and the String::from_utf8_lossy string's data pointer. This is later turned back into a Rust string by:

cxx/src/result.rs

Lines 61 to 66 in a396524

	let err = self.err;
	let slice = slice::from_raw_parts_mut(err.ptr.as_ptr(), err.len);
	let s = str::from_utf8_unchecked_mut(slice);
	Err(Exception {
	what: Box::from_raw(s),
	})

Since in general the length of string produced by from_utf8_lossy can be longer than the original string, this could end up truncating characters off the end of the exception message when printed from Rust code.