x11 generator: Don't crash when a switch is in a struct

x11 generator: Don't crash when a switch is in a struct

The size calculation for the struct will be wrong (too small) which looks
strange in the GUI when the children elements extend past the region
marked by their parent. But it doesn't seem to cause any actual problems,
there is only one request affected by this, and correct size calculation
(by recursing the switch) would take a fair amount of work.

Change-Id: I1847f736153daf59b8dbf3299005a772ffe9673a
Reviewed-on: https://code.wireshark.org/review/11107
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>