libpqtypes - libpqtypes: fixed possible null pointer access in

Log Message:
-----------
fixed possible null pointer access in PQparamExec and PQparamSendQuery. Both functions could have passed the native libpq counterparts a NULL command string

Modified Files:
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libpqtypes/src:
exec.c (r1.18 -> r1.19)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/libpqtypes/libpqtypes/src/exec.c?r1=1.18&r2=1.19)