Fix pg_dump to not emit invalid SQL for an empty operator class.

If an operator class has no operators or functions, and doesn't need
a STORAGE clause, we emitted "CREATE OPERATOR CLASS ... AS ;" which
is syntactically invalid.  Fix by forcing a STORAGE clause to be
emitted anyway in this case.

(At some point we might consider changing the grammar to allow CREATE
OPERATOR CLASS without an opclass_item_list.  But probably we'd want to
omit the AS in that case, so that wouldn't fix this pg_dump issue anyway.)

It's been like this all along, so back-patch to all supported branches.

Daniel Gustafsson, tweaked by me to avoid a dangling-pointer bug

Discussion: https://postgr.es/m/D9E5FC64-7A37-4F3D-B946-7E4FB468F88A@yesql.se

src/bin/pg_dump/pg_dump.c

@@ -12440,7 +12440,8 @@ dumpOpclass(Archive *fout, OpclassInfo *opcinfo)
 	i_opcfamilynsp = PQfnumber(res, "opcfamilynsp");
 	i_amname = PQfnumber(res, "amname");
 
-	opcintype = PQgetvalue(res, 0, i_opcintype);
+	/* opcintype may still be needed after we PQclear res */
+	opcintype = pg_strdup(PQgetvalue(res, 0, i_opcintype));
 	opckeytype = PQgetvalue(res, 0, i_opckeytype);
 	opcdefault = PQgetvalue(res, 0, i_opcdefault);
 	/* opcfamily will still be needed after we PQclear res */
@@ -12674,6 +12675,15 @@ dumpOpclass(Archive *fout, OpclassInfo *opcinfo)
 
 	PQclear(res);
 
+	/*
+	 * If needComma is still false it means we haven't added anything after
+	 * the AS keyword.  To avoid printing broken SQL, append a dummy STORAGE
+	 * clause with the same datatype.  This isn't sanctioned by the
+	 * documentation, but actually DefineOpClass will treat it as a no-op.
+	 */
+	if (!needComma)
+		appendPQExpBuffer(q, "STORAGE %s", opcintype);
+
 	appendPQExpBufferStr(q, ";\n");
 
 	appendPQExpBuffer(labelq, "OPERATOR CLASS %s",
@@ -12701,6 +12711,8 @@ dumpOpclass(Archive *fout, OpclassInfo *opcinfo)
 					opcinfo->dobj.namespace->dobj.name, opcinfo->rolname,
 					opcinfo->dobj.catId, 0, opcinfo->dobj.dumpId);
 
+	free(opcintype);
+	free(opcfamily);
 	free(amname);
 	destroyPQExpBuffer(query);
 	destroyPQExpBuffer(q);