Memory Access Violation on SANE's test backend

[Gonçalo MB]

During the sane-wasm build we are suppose to set number_of_devices to 0 (test backend). But I've discovered an issue with that option, see build.sh comment:

sane-wasm/build.sh

Lines 236 to 251 in 3100bc9

 # Set test backend's number of devices to 0. 
 # XXX: Oops what? There is some kind of invalid memory access on the backend 
 #      file deps/backends/backend/test.c, setting number_of_devices to 0 causes 
 #      some weird behavior and it's as if 1 device was selected. 
 #      I've looked briefly into this, but was unable to find the cause, 
 #      it's somehow related to how the option is read with: 
 #      read_option (line, "number_of_devices", ... 
 #      and then used on a for loop: 
 #      for (num = 0; num < init_number_of_devices; num++) 
 #      that loop should never run with init_number_of_devices = 0. 
 #      I've tested this config on my linux distribution version of SANE and 
 #      it triggers a Segmentation Fault so there is definitely a problem there. 
 # XXX: Setting it to -999 for now, fix this, and revert this to 0. 
 #      The fix should probably be pushed upstream to SANE itself: 
 #      https://gitlab.com/sane-project/backends/-/issues 
 sed -i "s/^number_of_devices .*/number_of_devices -999/g" "$PREFIX/etc/sane.d/test.conf" 

The issue is most likely on the file backends/backend/test.c.

The backends patch should be updated to include the fix: https://github.com/goncalomb/sane-wasm/tree/master/deps. And bonus points for fixing it upstream: https://gitlab.com/sane-project/backends/-/issues.