Silence compiler warnings on clang 21

Clang 21 shows some new compiler warnings, for example:

warning: variable 'dstsize' is uninitialized when passed as a const pointer argument here [-Wuninitialized-const-pointer]

The fix is to initialize the variables when they are defined.  This is
similar to, for example, the existing situation in gistKeyIsEQ().

Discussion: https://www.postgresql.org/message-id/flat/6604ad6e-5934-43ac-8590-15113d6ae4b1%40eisentraut.org

src/backend/access/common/toast_internals.c

@@ -135,7 +135,7 @@ toast_save_datum(Relation rel, Datum value,
 		char		data[TOAST_MAX_CHUNK_SIZE + VARHDRSZ];
 		/* ensure union is aligned well enough: */
 		int32		align_it;
-	}			chunk_data;
+	}			chunk_data = {0};	/* silence compiler warning */
 	int32		chunk_size;
 	int32		chunk_seq = 0;
 	char	   *data_p;

src/backend/access/gist/gistutil.c

@@ -157,7 +157,7 @@ gistMakeUnionItVec(GISTSTATE *giststate, IndexTuple *itvec, int len,
 {
 	int			i;
 	GistEntryVector *evec;
-	int			attrsize;
+	int			attrsize = 0;	/* silence compiler warning */
 
 	evec = (GistEntryVector *) palloc((len + 2) * sizeof(GISTENTRY) + GEVHDRSZ);
 
@@ -242,7 +242,7 @@ gistMakeUnionKey(GISTSTATE *giststate, int attno,
 		char		padding[2 * sizeof(GISTENTRY) + GEVHDRSZ];
 	}			storage;
 	GistEntryVector *evec = &storage.gev;
-	int			dstsize;
+	int			dstsize = 0;	/* silence compiler warning */
 
 	evec->n = 2;