Add runtime checks for number of query parameters passed to libpq functions.

Add runtime checks for number of query parameters passed to libpq functions.

The maximum number of parameters supported by the FE/BE protocol is 65535,
as it's transmitted as a 16-bit unsigned integer. However, the nParams
arguments to libpq functions are all of type 'int'. We can't change the
signature of libpq functions, but a simple bounds check is in order to make
it more clear what's going wrong if you try to pass more than 65535
parameters.

Per complaint from Jim Vanns.