tests: fcb: initialize buffers to avoid processing uninitialized values

Running the test under valgrind identified three places where an
uninitialized stack buffer was used as the source of data over which a
CRC was incidently calculated, causing an uninitialized value warning
in the CRC calculation.  Zero out the source buffers before using them
as a data source.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>