libdfp: Function left_justify fixed for PPC/DPD.

Added a check to left_justify function in order to catch when the
number justified would become a NaN data. This patch also adds tests
to verify this.
libdecnumber: Fix potentially out of the bound array access

decGetInt can potentially acess a out of the bounds value from array
DECPOWERS, this patch fixes the test to actually access the array.