Avoid out-of-bounds read in errfinish if error_stack_depth < 0.

If errordata_stack_depth < 0, we won't find that out and correct the
problem until CHECK_STACK_DEPTH() is invoked.  In the meantime,
elevel will be set based on an invalid read.  This is probably
harmless in practice, but it seems cleaner this way.

Xi Wang
Make NUM_TOCHAR_prepare and NUM_TOCHAR_finish macros declare "len".

Remove the variable from the enclosing scopes so that nothing can be
relying on it.  The net result of this refactoring is that we get rid
of a few unnecessary strlen() calls.

Original patch from Greg Jaskiewicz, substantially expanded by me.