Fix warning about change of sign: glyph_t is unsigned

qsgdistancefieldglyphnode_p.cpp(222): warning #68: integer conversion resulted in a change of sign

Use the value 0, which is reserved to mean "no texture"

Change-Id: I0bb135639c432ab08f6561c1d45f64e2d8f96dd7
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>