hdf5_ep

[svn-r24215] Bug fix: HDFFV-8271 Big.c error in address generated by random number.

Description:
There is a bug in the current big.c tests. In a nut-shell,
the writing part randomly picks 50 locations and write 4KB
from that location. Random location range is from byte 0
to end of dataset. There is the problem--if the random location
is greater than (end of dataset - 4KB), then error.

Solution:
Adjust the random number to be at least 4KB less than the end of the
dataset. That allows it write 4KB more from the generated starting
position.

Also put in a proper error message if unexpected switch values happened.

Tested: h5comittest plus jam serial build and test.