winegdb: Fix crash in empty first command

[Vitor T. Sessak]

Changelog:
winegdb: Fix crash in empty first command

Winedbg crashes when pressing enter as its first command.

This is my first patch, so don't mind telling me if I did anything wrong.

diff -ur a/programs/winedbg/debug.l b/programs/winedbg/debug.l
--- a/programs/winedbg/debug.l	Mon Mar  6 22:03:28 2006
+++ b/programs/winedbg/debug.l	Mon Mar  6 22:03:20 2006
@@ -34,7 +34,7 @@
 static int read_input(const char* pfx, char* buf, int size)
 {
     size_t      len;
-static char*  last_line = NULL;
+static char*  last_line = "\n";
 static size_t last_line_idx = 0;
 
     /* try first to fetch the remaining of an existing line */