Commit 9e2103f9 added support for zero-length tails by conditionally
creating the line object. However it kept using that object
unconditionally in some places (such as setLength), which results in
undefined references if an ArrowHelper is initially created with a
zero-size tail and then made longer by calling setLength().

This patch fixes this problem by always creating the tail object and
limiting its minimum length to 0. This also simplifies the code a bit.