Fix crash in json_file_check_for_errors with gfortran. Fixes #420

Author: jacobwilliams

src/json_file_module.F90

@@ -447,7 +447,7 @@ 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)
-    error_msg = tmp
+    if (present(error_msg)) error_msg = tmp
 #else
     call me%core%check_for_errors(status_ok,error_msg)
 #endif