Dropped self assignment when initialising variables in tokenize.c.

The 'int x=x' style statement do not make sense and cause problems with
some compilers.  Replace it with 'int x=0' to ensure the value
have a well known starting point.

Added -Werror=uninitialized -Winit-self to default GCC build to ensure
this style of statement do not sneak into the code again.

Fixes #2317