Avoid "D linker : (null)" with debug.ld.all for dlerror.

We only want to know when dlerror is actually set. The previous change
to this logic moved it so that we only show actual updates to dlerror,
not every string that might end up in dlerror's output. This change
ignores cases where we're _clearing_ dlerror (which happens on every
call to dlerror).

Bug: http://b/37287938
Test: ran tests
Change-Id: I0c30ee199dc76d9aea165c1d90f694ead488518b