Put back initialization of 'sslmode', to silence Coverity

Coverity pointed out that the function checks for conn->sslmode !=
NULL, which implies that it might be NULL, but later we access it
without a NULL-check anyway. It doesn't know that it is in fact always
initialized earlier, in conninfo_add_defaults(), and hence the
NULL-check is not necessary. However, there is a lot of distance
between conninfo_add_defaults() and pqConnectOptions2(), so it's not
surprising that it doesn't see that. Put back the initialization code,
as it existed before commit 05fd30c0e7, to silence the warning.

In the long run, I'd like to refactor the libpq options handling and
initalization code. It seems silly to strdup() and copy strings, for
things like sslmode that have a limited set of possible values; it
should be an enum. But that's for another day.

src/interfaces/libpq/fe-connect.c

@@ -1576,6 +1576,12 @@ pqConnectOptions2(PGconn *conn)
 		}
 #endif
 	}
+	else
+	{
+		conn->sslmode = strdup(DefaultSSLMode);
+		if (!conn->sslmode)
+			goto oom_error;
+	}
 
 	/*
 	 * validate sslnegotiation option, default is "postgres" for the postgres