Fortran: Fix COMPILER-9235/ac-constructor str len

Fixes COMPILER-9235 by setting maximal string length to the correct
variable. (Side remark: The code in the bug report is invalid as the array
constructor contains string literals of different lengths; use "-ansi" to
print a warning. As extension the compiler handles different lengths for
constant expressions.)

* src/crayf90/fe90/s_asg_expr.c (array_construct_semantics): Properly set the
  expression's string length.

Made by Tobias Burnus <burnus@net-b.de> for enzo.