Fix NPE in Field#getAnnotation

When the class whose field is to be inspected has no annotations at all,
at least my javac here (1.6.0_51 on MacOSX) does not produce any class
addendum.

Therefore, let's verify that the addendum is not null before proceeding.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>