[PATCH] PR fortran/67987 -- character lengths cannot be negative

[Steve Kargl]

The attach patch enforces the Fortran Standard's requirement
that character length must be great than or equal to zero.
The fix submitted here supercedes the fix for PR fortran/31250,
which silently converted a negative string length to zero.
In removing the fix for 31250, a regression occurred, because
a substring reference where the 'start' value is larger than 
'end' value results in a zero length string.  gfortran was computing
the length, but not checking for a negative value.  This has
be fixed by actually doing the check and resetting legnth to
zero. 

Built and tested on x86_64-*-freebsd.  OK to commit? 

2015-10-16  Steven G. Kargl  <kargl@gcc.gnu.org>

	* decl.c (char_len_param_value): Check for negative character length.
	Unwrap a nearby unlong line.
	* resolve.c (gfc_resolve_substring_charlen): If a substring would have
	a negative character length, set it to zero per the Fortran standard.
	Unwrap a nearby unlong line.
	(resolve_charlen): Check for negative character length.

2015-10-16  Steven G. Kargl  <kargl@gcc.gnu.org>

	gfortran.dg/char_length_2.f90: Update the testcase.

-- 
Steve
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