Wrong fallback logic for name() libbacktrace backend

[prbegd]

Fixes #212

https://github.com/ianlancetaylor/libbacktrace/blob/master/backtrace.h#L180 shows that the return value of backtrace_pcinfo depends on the return value of the callback function. But the current libbacktrace_full_callback always returns 0:

stacktrace/include/boost/stacktrace/detail/libbacktrace_impls.hpp

Lines 50 to 60 in 4f8cabb

	inline int libbacktrace_full_callback(void *data, uintptr_t /*pc*/, const char *filename, int lineno, const char *function) {
	pc_data& d = *static_cast<pc_data*>(data);
	if (d.filename && filename) {
	*d.filename = filename;
	}
	if (d.function && function) {
	*d.function = function;
	}
	d.line = static_cast<std::size_t>(lineno);
	return 0;
	}

This leads the invocation of backtrace_pcinfo to always return 0, which makes the logical OR expression always evaluate the right hand side backtrace_syminfo:

stacktrace/include/boost/stacktrace/detail/libbacktrace_impls.hpp

Lines 177 to 191 in 4f8cabb

	::backtrace_pcinfo(
	state,
	reinterpret_cast<uintptr_t>(addr),
	boost::stacktrace::detail::libbacktrace_full_callback,
	boost::stacktrace::detail::libbacktrace_error_callback,
	&data
	)
	||
	::backtrace_syminfo(
	state,
	reinterpret_cast<uintptr_t>(addr),
	boost::stacktrace::detail::libbacktrace_syminfo_callback,
	boost::stacktrace::detail::libbacktrace_error_callback,
	&data
	);

Therefore, the result from backtrace_pcinfo is always overwritten by the result from backtrace_syminfo.

As a solution, libbacktrace_full_callback will now return 1 only when there's valid data.

[apolukhin]

Many thanks for the PR! Great job!