generator: Check parameters are not NULL (RHBZ#501893).

generator: Check parameters are not NULL (RHBZ#501893).

This adds additional tests to check that several types of parameter
including String are not NULL when passed to the C functions.
Previously this would cause a segfault inside libguestfs.  With
this change, you get an error message / exception.

Of the possible pointer parameters, only OptString is now permitted
to be NULL.

This change does not affect the Perl bindings.  This is because Perl
XS code was already adding similar checks if you passed undef into
a parameter expecting a string.