PEM parsing: check last error instead of first

[botovq]

Regression introduced in #2120. Ideally, tests would not leave other errors behind.

Fixes #2146

[alex]

Hmm, my impression when I wrote that code is that [0] is the item most recently added to the stack. Is that not right?

Using last seems to presume other errors will be added there after the no start line error.

[botovq]

It uses Err_get_error_line_data() which is the oldest error. The code you replaced used ERR_peek_last_error() which is the latest error. Running this in a loop has been stable for more than an hour while it would previously crash after a few minutes.

rust-openssl/openssl/src/error.rs

Lines 367 to 377 in 06143eb

	unsafe extern "C" fn ERR_get_error_all(
	file: *mut *const c_char,
	line: *mut c_int,
	func: *mut *const c_char,
	data: *mut *const c_char,
	flags: *mut c_int,
	) -> ErrType {
	let code = ffi::ERR_get_error_line_data(file, line, data, flags);
	*func = ffi::ERR_func_error_string(code);
	code
	}

[alex]

😱. We probably should make that unambigiously clear in the docs. But for now: thank you!