from  the man-page of XF86VidModeGetMonitor:

  MONITOR FUNCTIONS
       Information  known  to the server about the monitor is returned by the XF
       function.  The hsync and vsync fields each point to an array of XF86VidMo
       tures.   The arrays contain nhsync and nvsync elements, respectively.  Th
       will be equal if a discreate value was given in the XF86Config file.

       The vendor, model, hsync, and vsync fields  point  to  dynamically  alloc
       should be freed by the caller.

dynamically allocated ??
not really

from XF86VMode.c

    if (rep.vendorLength)
        _XReadPad(dpy, monitor->vendor, rep.vendorLength);
    else
        monitor->vendor = "";
    if (rep.modelLength)
        _XReadPad(dpy, monitor->model, rep.modelLength);
    else
        monitor->model = "";
        
a pointer to a static "" is not dynamically!!
so

    XF86VidModeGetMonitor(dpy, DefaultScreen(dpy), &monitor)

    XFree(monitor.vendor);   -> crash
    XFree(monitor.model);    -> crash



were does the problem arise: xdpyinfo

   [...]
   XFree86-VidModeExtension version 2.2 opcode: 134, base error: 130
     Monitor Information:
       Vendor: , Model: 
       Num hsync: 1, Num vsync: 1
       hsync range 0:  30.00 -  70.00
       vsync range 0:  47.00 -  85.00
   *** glibc detected *** ./xdpyinfo: munmap_chunk(): invalid pointer: 0xb7ec794