Fix miscompile of for loop with multiple declarations.

8cc miscompiled for loop if its initializer is variable
definition that defines multiple variables. E.g.

  for (int i = 0, j = 0; ...)

Parser had a bug that only the first variable was defined.
Simplify.
Do not allow out-of-bound access to Vector.

Returning NULL instead of raising an error is sometimes
convenient, but it could hide subtle bugs. Do not allow
out-of-bound access to a vector.