fix for Creating Snippet from Selection crashes Notepad++ #14

[chcg]

Issue #14 with selections is caused by a buffer overrun issue in:

Snippet::SetBeforeSelection()

with the handling of MultiByteToWideChar (https://docs.microsoft.com/en-us/windows/desktop/api/stringapiset/nf-stringapiset-multibytetowidechar) not using size of chars correctly.

[chcg]

@ffes What do you think about #16:

Callstack for the problem of adding a new entry with name "test" and empty "before cursor" entry:

 	KernelBase.dll!RaiseException�()	Unbekannt
 	[Externer Code]	
>	NppSnippets.dll!SqliteStatement::SaveRecord() Zeile 269	C++
 	NppSnippets.dll!Snippet::SaveToDB(bool autoOpen) Zeile 247	C++
 	NppSnippets.dll!OnOK(HWND__ * hDlg) Zeile 110	C++
 	NppSnippets.dll!DlgProc(HWND__ * hDlg, unsigned int message, unsigned __int64 wParam, __int64 lParam) Zeile 247	C++
 	[Externer Code]	
 	NppSnippets.dll!ShowEditSnippetDlg(Snippet * pSnippet) Zeile 280	C++
 	NppSnippets.dll!OnSnippetAdd(HWND__ * hWnd) Zeile 619	C++
 	NppSnippets.dll!OnCommand(HWND__ * hWnd, int ResID, int msg) Zeile 1108	C++
 	NppSnippets.dll!DlgProcedure(HWND__ * hWnd, unsigned int message, unsigned __int64 wParam, __int64 lParam) Zeile 1242	C++
 	[Externer Code]	
 	notepad++.exe!WinMain(HINSTANCE__ * hInstance, HINSTANCE__ * __formal, char * __formal, int __formal) Zeile 566	C++
 	[Externer Code]	

this is caused by sql table definition for snippets:

	BeforeSelection TEXT NOT NULL,

and SqliteStatement::Bind(int col, const WCHAR* val)

	else if (wcslen(val) == 0)
	{
		res = sqlite3_bind_null(_stmt, col);
	}

, which sets empty input to null.

[ffes]

Thanks for your contribution