tools: crash on exit from level testing

[dbartolini]

The Level Editor sometimes crashes due to bad messages received at the end of a level testing session (F5):

json: {"type":"message","severity":"info","system":"device","message":"BGFX     GL_FLOAT_VEC2 a_texcoord0 is at location 2"}
device: BGFX Shutdown...
device: BGFX render thread exit
device: BGFX Shutdown complete.
json: 
**
ERROR:../../../tools/core/json/json.vala:220:crown_json_parse: assertion failed: (false)
Bail out! ERROR:../../../tools/core/json/json.vala:220:crown_json_parse: assertion failed: (false)

A correct exit from testing looks like this:

json: {"type":"message","severity":"info","system":"device","message":"BGFX     attr 10: Attrib::TexCoord0    num 2, type 4, norm [ ], asint [ ], offset 12"}
device: BGFX Shutdown...
json: {"type":"message","severity":"info","system":"device","message":"BGFX Shutdown..."}
device: BGFX render thread exit
json: {"type":"message","severity":"info","system":"device","message":"BGFX render thread exit"}
device: BGFX Shutdown complete.
json: {"type":"message","severity":"info","system":"device","message":"BGFX Shutdown complete."}
json: {"type":"remove_file","path":"_level_editor_test.level"}
json: {"type":"remove_file","path":"_level_editor_test.package"}