crash when member variable declared to itself

[Nutriz]

Operating system or device, Godot version, GPU Model and driver (if graphics related):
Windows 10 64 bits
Godot 3.0 alpha1

Issue description:
Godot crash when I try to access auto completion on a member variable declared to itself

Steps to reproduce:
In any kind of script attached to a node, create a member variable like this : var test = test
In a function, (_ready for my test), type test. (the dot is important)
After a few second I got a message saying godot must close

[hpvb]

Yeah, the editor runs out of stack:

#0  0x00007f1764270ccf in mcount () from /lib64/libc.so.6
#1  0x00000000008eebcc in Vector<wchar_t>::_unref (this=0x7fffff7ff180, p_data=0x0) at core/vector.h:189
#2  0x00000000008f5f72 in Vector<wchar_t>::_copy_from (this=this@entry=0x7fffff7ff180, p_from=...) at core/vector.h:380
#3  0x0000000000b28079 in Vector<wchar_t>::Vector (p_from=..., this=0x7fffff7ff180) at core/vector.h:413
#4  String::String (this=0x7fffff7ff180) at core/ustring.h:62
#5  GDCompletionContext::GDCompletionContext (this=0x7fffff7ff160) at modules/gdscript/gd_editor.cpp:401
#6  _guess_identifier_type (p_for_indexing=false, r_type=..., p_identifier=..., p_line=6, context=...) at modules/gdscript/gd_editor.cpp:1109
#7  _guess_expression_type (context=..., p_node=0x776d470, p_line=p_line@entry=7, r_type=..., p_for_indexing=p_for_indexing@entry=false) at modules/gdscript/gd_editor.cpp:571
#8  0x0000000000b294d1 in _guess_identifier_type (p_for_indexing=false, r_type=..., p_identifier=..., p_line=6, context=...) at modules/gdscript/gd_editor.cpp:1189
#9  _guess_expression_type (context=..., p_node=0x776d470, p_line=p_line@entry=7, r_type=..., p_for_indexing=p_for_indexing@entry=false) at modules/gdscript/gd_editor.cpp:571
#10 0x0000000000b294d1 in _guess_identifier_type (p_for_indexing=false, r_type=..., p_identifier=..., p_line=6, context=...) at modules/gdscript/gd_editor.cpp:1189

until forever

Looks like we need some loop detection here.

[reduz]

@bojidar-bg or someone else would like to give it a spin?
seems like a simple crash to work around in gd_editor.cpp auto completion code.