Replace some fancy stuff with simple function
memset not required as struct is already initialized to 0.

Moreover, memset causes loss of sDecParam.sVideoProperty.size
that was set previously.
Fix typo which ignored dec options file in dec testapp

Options file was not being taken because it was wrongly being taken as output
file for decoder.
Improve command line arguments handling in dec testapp

Problems fixed:
1. No longer segfaults if an option is specified without an accompanying argument.

2. Wrongly incrementing i by 3 instead of by 2 while parsing command line options.