Null pointer dereferencies in src/v4l.c, src/remote.c, src/screen.c, src/user.c

[shaue188]

Additional context

Our company uses x11vnc in our product. I am working on static analysis, and one of my tasks was to scan x11vnc package. During the scan, I found some null pointer dereferencies in code:

Describe errors

1. https://github.com/LibVNC/x11vnc/blob/master/src/v4l.c#L721
   Pointer p get the value in https://github.com/LibVNC/x11vnc/blob/master/src/v4l.c#L716 using strpbrk and can be NULL. Need to check that pointer p is not NULL before dereferencing as in https://github.com/LibVNC/x11vnc/blob/master/src/v4l.c#L717

2. https://github.com/LibVNC/x11vnc/blob/master/src/remote.c#L184
   Pointer dpy can be NULL, need to check that pointer p is not NULL before dereferencing as in https://github.com/LibVNC/x11vnc/blob/master/src/remote.c#L121

3. https://github.com/LibVNC/x11vnc/blob/master/src/remote.c#L569
   In https://github.com/LibVNC/x11vnc/blob/master/src/remote.c#L574 checks that screen is not NULL, while in https://github.com/LibVNC/x11vnc/blob/master/src/remote.c#L569 checks that screen->httpDir is not NULL, which can lead to a null pointer dereference if screen is NULL

4. https://github.com/LibVNC/x11vnc/blob/master/src/screen.c#L2943
   Pointer fb can be NULL, need to check that pointer fb is not NULL before dereferencing.

5. https://github.com/LibVNC/x11vnc/blob/master/src/screen.c#L4506
   Pointer screen can be NULL, so screen->deferUpdateTime can result in a null pointer dereference. Need to check that pointer screen is not NULL before dereferencing as in https://github.com/LibVNC/x11vnc/blob/master/src/screen.c#L4485

6. https://github.com/LibVNC/x11vnc/blob/master/src/user.c#L2637
   Pointer keep_unixpw_user can be NULL. Need to check that pointer keep_unixpw_user is not NULL before dereferencing as in https://github.com/LibVNC/x11vnc/blob/master/src/user.c#L2565