loader: Handle unavailable VBE during mode lookup

When VBE is unavailable, vbe_init() frees the controller information
buffer and sets vbe to NULL. Console initialization still calls
vbe_default_mode(), which reaches an unguarded memset() in
vbe_find_mode_xydm(). Under BTX, the null write can overwrite the
loader's own code and cause a later protection fault.

Guard vbe_default_mode() with vbe_check(), as the other public
functions in vbe.c already do, and return 0 to indicate that no
graphics mode is available. This allows console initialization to
continue on headless systems.

Reviewed by:	rmoeller, imp
MFC ater:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D61145

stand/i386/libi386/vbe.c

@@ -1125,6 +1125,9 @@ vbe_default_mode(void)
 	int modenum;
 	uint32_t width, height;
 
+	if (!vbe_check())
+		return (0);
+
 	modenum = 0;
 	vbe_get_max_resolution(&width, &height);
 	if (width != 0 && height != 0)