Fix crash with stepi, no debug info, and "set debug infrun 1"

Fix crash with stepi, no debug info, and "set debug infrun 1"

A stepi in a function without debug info with "set debug infrun 1"
crashes GDB since commit c8353d682f69 (gdb/infrun: some extra infrun
debug print statements), due to a reference to
"tp->current_symtab->filename" when tp->current_symtab is null.

This commit adds the missing null check.  The output in this case
becomes:

  [infrun] set_step_info: symtab = <null>, line = 0, step_frame_id = {stack=0x7fffffffd980,code=0x0000000000456c30,!special}, step_stack_frame_id = {stack=0x7fffffffd980,code=0x0000000000456c30,!special}

Change-Id: I5171a9d222bc7e15b9dba2feaba7d55c7acd99f8