When associating a variable of type character, if the length of the
target isn't known at compile time, generate an error. See PR 83344
for more details.

Regtested on x86_64-pc-linux-gnu.

gcc/fortran/ChangeLog:

2018-02-01  Janne Blomqvist  <jb@gcc.gnu.org>

	PR 83975
	PR 83344
	* resolve.c (resolve_assoc_var): Generate an error if
	target length unknown.

From-SVN: r257310