Suppress maybe-uninitialized warning.

Following commit e035863c9a, building with -O0 began triggering
warnings about potentially uninitialized 'workbuf' usage. While
theoretically the initialization isn't necessary since VARDATA()
doesn't access the contents of the pointed-to object, this commit
explicitly initializes the workbuf variable to suppress the warning.

Buildfarm members adder and flaviventris have shown the warning.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAD21AoCOZxfqnNgfM5yVKJZYnOq5m2Q96fBGy1fovEqQ9V4OZA@mail.gmail.com

src/backend/storage/large_object/inv_api.c

@@ -561,7 +561,7 @@ inv_write(LargeObjectDesc *obj_desc, const char *buf, int nbytes)
 		char		data[LOBLKSIZE + VARHDRSZ];
 		/* ensure union is aligned well enough: */
 		int32		align_it;
-	}			workbuf;
+	}			workbuf = {0};
 	char	   *workb = VARDATA(&workbuf.hdr);
 	HeapTuple	newtup;
 	Datum		values[Natts_pg_largeobject];
@@ -752,7 +752,7 @@ inv_truncate(LargeObjectDesc *obj_desc, int64 len)
 		char		data[LOBLKSIZE + VARHDRSZ];
 		/* ensure union is aligned well enough: */
 		int32		align_it;
-	}			workbuf;
+	}			workbuf = {0};
 	char	   *workb = VARDATA(&workbuf.hdr);
 	HeapTuple	newtup;
 	Datum		values[Natts_pg_largeobject];