Fix crash on 64 bit Linux in command line argument parsing

[ptriller]

On my Ubuntu 16.04 the command line parsing fails with SIGSEGV (see below)

The code to fix it was inspired by https://github.com/gotk3/gotk3 and ported to you. It seems you already had the necessary C functions, so the patch is minimal.

ptriller@nuc:~/go-workspace/src/github.com/mattn/go-gtk/example/demo$ go build demo.go 
ptriller@nuc:~/go-workspace/src/github.com/mattn/go-gtk/example/demo$ gdb ./demo 
GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.04) 7.11.1
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Reading symbols from ./demo...done.
warning: File "/home/ptriller/Programs/go/src/runtime/runtime-gdb.py" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load".
To enable execution of this file add
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(gdb) run
Starting program: /home/ptriller/go-workspace/src/github.com/mattn/go-gtk/example/demo/demo 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff1017700 (LWP 27922)]
[New Thread 0x7ffff0816700 (LWP 27923)]
[New Thread 0x7ffff0015700 (LWP 27924)]
[New Thread 0x7fffef814700 (LWP 27925)]

Thread 1 "demo" received signal SIGSEGV, Segmentation fault.
0x00007ffff76884bd in g_path_get_basename () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
(gdb) bt
#0  0x00007ffff76884bd in g_path_get_basename () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#1  0x00007ffff76a6531 in g_option_context_parse () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#2  0x00007ffff71364b8 in gtk_parse_args () from /usr/lib/x86_64-linux-gnu/libgtk-x11-2.0.so.0
#3  0x00007ffff7136539 in gtk_init () from /usr/lib/x86_64-linux-gnu/libgtk-x11-2.0.so.0
#4  0x0000000000495050 in runtime.asmcgocall () at /home/ptriller/Programs/go/src/runtime/asm_amd64.s:590
#5  0x00000000004421ad in runtime.cgocall (fn=0x7fffffffdb30, arg=0x9f1620 <runtime.g0>, ~r2=-9440) at /home/ptriller/Programs/go/src/runtime/cgocall.go:115
#6  0x00000000009f1600 in runtime.work ()
#7  0x00007fffffffdb30 in ?? ()
#8  0x00000000009f1620 in ?? ()
#9  0x00007fffffffdb20 in ?? ()
#10 0x000000000046c744 in runtime.mstart () at /home/ptriller/Programs/go/src/runtime/proc.go:1096
#11 0x00000000004931a5 in runtime.rt0_go () at /home/ptriller/Programs/go/src/runtime/asm_amd64.s:156
#12 0x0000000000000001 in ?? ()
#13 0x00007fffffffdc38 in ?? ()
#14 0x0000000000000001 in ?? ()
#15 0x00007fffffffdc38 in ?? ()
#16 0x0000000000000000 in ?? ()
(gdb) quit
A debugging session is active.

	Inferior 1 [process 27918] will be killed.

Quit anyway? (y or n) y
ptriller@nuc:~/go-workspace/src/github.com/mattn/go-gtk/example/demo$ 

[mattn]

Thank you