[NTVDM:VBE]: Don't crash if the video mode is invalid.

svn path=/trunk/; revision=73028

reactos/subsystems/mvdm/ntvdm/bios/bios32/vbe.c

@@ -356,6 +356,7 @@ static VOID VbeSetExtendedRegisters(PSVGA_REGISTERS Registers)
 BOOLEAN WINAPI VbeSetExtendedVideoMode(BYTE ModeNumber)
 {
     PCVBE_MODE Mode = VbeGetModeByNumber(ModeNumber);
+    if (Mode == NULL) return FALSE;
 
     /* At this point, Mode->Registers shouldn't be NULL unless the mode is unimplemented */
     if (Mode->Registers == NULL)