orc: Don't set memory protection if there's no code

If the size of the JIT code is 0, there's no code and the *mem is
uninitialized. This can happen when orcc.exe is used to generate
backup C code.

Part-of: <gstreamer/orc!44>
4 jobs for fix-orcc-usage-windows in 3 minutes and 26 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #3403304
debian sid

00:01:02

passed #3403307
gstreamer-windows 1809 windows docker
msys2

00:03:24

passed #3403305
gstreamer-windows 1809 windows docker
vs2017 amd64

00:00:57

passed #3403306
gstreamer-windows 1809 windows docker
vs2017 x86

00:00:57