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

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