a minor change to work around a possible compiler bug in Gfortran

Fixes #597

Author: jacobwilliams

src/json_file_module.F90

@@ -449,7 +449,7 @@ subroutine json_file_check_for_errors(me,status_ok,error_msg)
 #if defined __GFORTRAN__
     character(kind=CK,len=:),allocatable :: tmp  !! workaround for gfortran bugs
     call me%core%check_for_errors(status_ok,tmp)
-    if (present(error_msg)) error_msg = tmp
+    if (present(error_msg)) call move_alloc(tmp, error_msg)
 #else
     call me%core%check_for_errors(status_ok,error_msg)
 #endif