Fix the stack top fetching in ws_processor_prepare_args()

This patch fixes the stack top fetching in the
ws_processor_prepare_args() function. The problem was that the pushing
to the stack changes the stack and _realloc()_s it, if required. That
causes the stack object to be located in another position, which caused
everything to fail.

Suggested-by: Julian Ganz <julian.ganz@hs-furtwangen.de>
Add string test files
Add string test for initialization
Add test for checking empty string length
Add test for catting empty strings and checking their length afterwards
Add preparation for empty multicat string test
Add test for empty duplication of string objects
Add test for empty string cmp()
Add test for getting empty string as raw
Add test for setting string from literal
Add test for setting string from buffer on heap
Add setup/teardown functions for more complex string testing
Add string ncmp test
Add substr testing
Add string cmp test
Fix unlocking order in ws_string_cmp
Merge pull request #341 from matthiasbeyer/fix-action_processor

Fix the stack top fetching in ws_processor_prepare_args()
Fix bugs in ws_string_raw()

The error variable has to be reset and we can get the length of the
string by calling the function with NULL parameters here.
Add missing NULL-checks
Fix ws_string_set_from_raw()