HPCC-12107 Segmentation fault when supplying an incorrect Wuid to DATASET(WORKUNIT(...


Add code to check and convert 'w' to 'W' into secOpenWorkUnit()
Add code to check first character of the WUID into same function

Add code to check the return value of factory->openWorkUnit() to avoid
SEGFAULT caused by zero pointer.

Signed-off-by: Attila.Vamos <attila.vamos@gmail.com>
return after fail (which throws exception) is superfluous.