Suppress remaining -Waddress warnings from recent gcc versions.

Still an exercise in satisfying pedants.

src/interfaces/libpq/pqexpbuffer.h

@@ -59,6 +59,14 @@ typedef PQExpBufferData *PQExpBuffer;
 #define PQExpBufferBroken(str)	\
 	((str) == NULL || (str)->maxlen == 0)
 
+/*------------------------
+ * Same, but for use when using a static or local PQExpBufferData struct.
+ * For that, a null-pointer test is useless and may draw compiler warnings.
+ *------------------------
+ */
+#define PQExpBufferDataBroken(buf)	\
+	((buf).maxlen == 0)
+
 /*------------------------
  * Initial size of the data buffer in a PQExpBuffer.
  * NB: this must be large enough to hold error messages that might