preprocessor: Fix ICE with too long line in fmtwarn [PR96935]

preprocessor: Fix ICE with too long line in fmtwarn [PR96935]

Here we ICE in char_span::subspan because the offset it gets is -1.
It's -1 because get_substring_ranges_for_loc gets a location whose
column was 0.  That only happens in testcases like the attached where
we're dealing with extremely long lines (at least 4065 chars it seems).
This does happen in practice, though, so it's not just a theoretical
problem (e.g. when building the SU2 suite).

Fixed by checking that the column get_substring_ranges_for_loc gets is
sane, akin to other checks in that function.

gcc/ChangeLog:

	PR preprocessor/96935
	* input.c (get_substring_ranges_for_loc): Return if start.column
	is less than 1.

gcc/testsuite/ChangeLog:

	PR preprocessor/96935
	* gcc.dg/format/pr96935.c: New test.