Adding one-line exit statement for uninitialized isize variable that affected jf_test_6 with NAG compiler

Author: kmanalo

src/json_module.f90

@@ -4289,6 +4289,10 @@ subroutine get_current_line_from_file(iunit,line)
             if (istat==0) then
                 line = line//chunk
             else
+                !avoid a comparison to isize if EOF is at the start of object
+                ![some compilers will leave isize uninitialized for this case]
+                if (index(err_message, &
+                    "Unexpected end of file while parsing start of object") > 0) exit
                 if (isize>0 .and. isize<=n_chunk) line = line//chunk(1:isize)
                 exit
             end if