Fix various instances of undefined behavior

Mostly this involves checking for NULL pointer before doing operations
that add a non-zero offset.

The exception is an overflow warning in heap_fetch_toast_slice(). This
was caused by unneeded parentheses forcing an expression to be
evaluated to a negative integer, which then got cast to size_t.

Per clang 21 undefined behavior sanitizer.

Backpatch to all supported versions.

Co-authored-by: Alexander Lakhin <exclusion@gmail.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/777bd201-6e3a-4da0-a922-4ea9de46a3ee@gmail.com
Backpatch-through: 14

contrib/pg_trgm/trgm_gist.c

@@ -699,10 +699,13 @@ gtrgm_penalty(PG_FUNCTION_ARGS)
 	if (ISARRKEY(newval))
 	{
 		char	   *cache = (char *) fcinfo->flinfo->fn_extra;
-		TRGM	   *cachedVal = (TRGM *) (cache + MAXALIGN(siglen));
+		TRGM	   *cachedVal = NULL;
 		Size		newvalsize = VARSIZE(newval);
 		BITVECP		sign;
 
+		if (cache != NULL)
+			cachedVal = (TRGM *) (cache + MAXALIGN(siglen));
+
 		/*
 		 * Cache the sign data across multiple calls with the same newval.
 		 */

src/backend/access/heap/heaptoast.c

@@ -770,7 +770,7 @@ heap_fetch_toast_slice(Relation toastrel, Oid valueid, int32 attrsize,
 			chcpyend = (sliceoffset + slicelength - 1) % TOAST_MAX_CHUNK_SIZE;
 
 		memcpy(VARDATA(result) +
-			   (curchunk * TOAST_MAX_CHUNK_SIZE - sliceoffset) + chcpystrt,
+			   curchunk * TOAST_MAX_CHUNK_SIZE - sliceoffset + chcpystrt,
 			   chunkdata + chcpystrt,
 			   (chcpyend - chcpystrt) + 1);
 

src/backend/utils/adt/multirangetypes.c

@@ -484,8 +484,9 @@ multirange_canonicalize(TypeCacheEntry *rangetyp, int32 input_range_count,
 	int32		output_range_count = 0;
 
 	/* Sort the ranges so we can find the ones that overlap/meet. */
-	qsort_arg(ranges, input_range_count, sizeof(RangeType *), range_compare,
-			  rangetyp);
+	if (ranges != NULL)
+		qsort_arg(ranges, input_range_count, sizeof(RangeType *),
+				  range_compare, rangetyp);
 
 	/* Now merge where possible: */
 	for (i = 0; i < input_range_count; i++)

src/backend/utils/sort/sharedtuplestore.c

@@ -325,7 +325,8 @@ sts_puttuple(SharedTuplestoreAccessor *accessor, void *meta_data,
 
 	/* Do we have space? */
 	size = accessor->sts->meta_data_size + tuple->t_len;
-	if (accessor->write_pointer + size > accessor->write_end)
+	if (accessor->write_pointer == NULL ||
+		accessor->write_pointer + size > accessor->write_end)
 	{
 		if (accessor->write_chunk == NULL)
 		{