libstdc++: Fix out-of-bounds read in format string "{:{}." [PR110974]

libstdc++-v3/ChangeLog:

	PR libstdc++/110974
	* include/std/format (_Spec::_S_parse_width_or_precision): Check
	for empty range before dereferencing iterator.
	* testsuite/std/format/string.cc: Check for expected exception.
	Fix expected exception message in test_pr110862() and actually
	call it.