Prevent crash from unaligned memory access on sparc64, etc.

Fixes Bugzilla #602. I suspect this will crash elsewhere if the system
 actually had modelines to report, though.

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%403204

src/video/Xext/Xxf86vm/XF86VMode.c

@@ -214,6 +214,12 @@ SDL_NAME(XF86VidModeGetGamma)(Display *dpy, int screen, SDL_NAME(XF86VidModeGamm
     return True;
 }
 
+/* this is to prevent an unaligned memory write on CPUs that need that. */
+static void zap_ptr(char *ptr, size_t size)
+{
+    memset(ptr, '\0', size);
+}
+
 Bool
 SDL_NAME(XF86VidModeGetModeLine)(dpy, screen, dotclock, modeline)
     Display* dpy;
@@ -284,7 +290,7 @@ SDL_NAME(XF86VidModeGetModeLine)(dpy, screen, dotclock, modeline)
 	}
 	_XRead(dpy, (char*)modeline->private, modeline->privsize * sizeof(INT32));
     } else {
-	modeline->private = NULL;
+	zap_ptr((char *)&modeline->private, sizeof(modeline->private))
     }
     UnlockDisplay(dpy);
     SyncHandle();