Done clean-up in prep stmt API functions:
1) Removed some checks that were performed only in debug version
were making debug version more tolerable to user errors than 
production (and thus caused problems for example masking some
bugs). 
2) Also removed some other checks to make prep stmt API 
consistent with the rest of C API (this also in line with 
general politics - make checks in only those places where 
errors are very common and hard to spot).