Fix join_path_components() to not add a leading slash when joining to an

initial null string.

Per report from Robert Haas in testing psql \ir.

src/port/path.c

@@ -212,7 +212,8 @@ join_path_components(char *ret_path,
 	}
 	if (*tail)
 		snprintf(ret_path + strlen(ret_path), MAXPGPATH - strlen(ret_path),
-				 "/%s", tail);
+				/* only add slash if there is something already in head */
+				 "%s%s", head[0] ? "/" : "", tail);
 }