rustc: Work around a segmentation fault

rustc: Work around a segmentation fault

On Linux/Mac, I got a segmentation fault:

  (gdb) bt
  #0  0x00000000007519af in glue_take584 ()
  #1  0x00000000006d4bec in
    back::rpath::get_rpath_flags::_3899df2ca513c603 ()
  #2  0x00000000006c7655 in back::link::link_binary::_7afde00a9791031c ()
  #3  0x00000000007d3ff5 in driver::rustc::compile_input::thunk9212 ()
  #4  0x0000000000710f24 in driver::rustc::time::_3e691b2a4ba58aee ()
  #5  0x000000000071a79d in
    driver::rustc::compile_input::_7b4a41b87c18e034 ()
  #6  0x000000000072f0a9 in driver::rustc::main::_cd8b8c8185af3dee ()
  #7  0x000000000072f1ed in _rust_main ()
  #8  0x00007ffff7e6e146 in task_start_wrapper (a=<optimized out>) at
    ../src/rt/rust_task.cpp:176

The variable `output` or `out_filename` becomes (null) after the definition
of `fn unlib`. Move the function defintion to the beginning seems
prevent the crash on Linux.