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.

src/interfaces/libpq/fe-exec.c

@@ -1113,6 +1113,7 @@ PQsendQuery(PGconn *conn, const char *query)
 	if (!PQsendQueryStart(conn))
 		return 0;
 
+	/* check the argument */
 	if (!query)
 	{
 		printfPQExpBuffer(&conn->errorMessage,
@@ -1170,12 +1171,19 @@ PQsendQueryParams(PGconn *conn,
 	if (!PQsendQueryStart(conn))
 		return 0;
 
+	/* check the arguments */
 	if (!command)
 	{
 		printfPQExpBuffer(&conn->errorMessage,
 						libpq_gettext("command string is a null pointer\n"));
 		return 0;
 	}
+	if (nParams < 0 || nParams > 65535)
+	{
+		printfPQExpBuffer(&conn->errorMessage,
+						libpq_gettext("number of parameters must be between 0 and 65535\n"));
+		return 0;
+	}
 
 	return PQsendQueryGuts(conn,
 						   command,
@@ -1203,19 +1211,25 @@ PQsendPrepare(PGconn *conn,
 	if (!PQsendQueryStart(conn))
 		return 0;
 
+	/* check the arguments */
 	if (!stmtName)
 	{
 		printfPQExpBuffer(&conn->errorMessage,
 						libpq_gettext("statement name is a null pointer\n"));
 		return 0;
 	}
-
 	if (!query)
 	{
 		printfPQExpBuffer(&conn->errorMessage,
 						libpq_gettext("command string is a null pointer\n"));
 		return 0;
 	}
+	if (nParams < 0 || nParams > 65535)
+	{
+		printfPQExpBuffer(&conn->errorMessage,
+						libpq_gettext("number of parameters must be between 0 and 65535\n"));
+		return 0;
+	}
 
 	/* This isn't gonna work on a 2.0 server */
 	if (PG_PROTOCOL_MAJOR(conn->pversion) < 3)
@@ -1298,12 +1312,19 @@ PQsendQueryPrepared(PGconn *conn,
 	if (!PQsendQueryStart(conn))
 		return 0;
 
+	/* check the arguments */
 	if (!stmtName)
 	{
 		printfPQExpBuffer(&conn->errorMessage,
 						libpq_gettext("statement name is a null pointer\n"));
 		return 0;
 	}
+	if (nParams < 0 || nParams > 65535)
+	{
+		printfPQExpBuffer(&conn->errorMessage,
+						libpq_gettext("number of parameters must be between 0 and 65535\n"));
+		return 0;
+	}
 
 	return PQsendQueryGuts(conn,
 						   NULL,	/* no command to parse */