lint: fix bugs in dprint_node in debug mode (since today)

The node was dereferenced before the null check.  GCC 5.5 didn't warn
about this obvious bug, not even with -Wall -Wextra -O2.  Such a case
didn't occur though in the few tests that this function was used in.

The indentation for the nested nodes only needs to be set for a few
lines of code, make this region as small as possible.

There are nodes that use both tn_left and tn_right, even though they are
not defined as binary operators.  An example is CALL, for which tn_left
is the address of the function name and tn_right, which are the
arguments, linked via PUSH nodes.  CALL is not a binary operator since
it doesn't do any calculations with its arguments.

(rillig)