BUG #17830: Incorrect memory access in trgm_regexp

The following bug has been logged on the website:

Bug reference:      17830
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 15.2
Operating system:   Ubuntu 22.04
Description:

When the following script executed:
CREATE EXTENSION pg_trgm;
CREATE TABLE t(t text);
CREATE INDEX t_idx_gin ON t USING gin (t gin_trgm_ops);
SELECT * FROM t WHERE t ~ '.*$x';

valgrind detects an invalid memory read:
==00:00:00:04.044 873608== Invalid read of size 4
==00:00:00:04.044 873608==    at 0x486B907: packGraph (trgm_regexp.c:2070)
==00:00:00:04.044 873608==    by 0x486C41E: createTrgmNFAInternal
(trgm_regexp.c:621)
==00:00:00:04.044 873608==    by 0x486C5CA: createTrgmNFA
(trgm_regexp.c:558)
==00:00:00:04.044 873608==    by 0x4865EEC: gin_extract_query_trgm
(trgm_gin.c:115)
==00:00:00:04.044 873608==    by 0x718F04: FunctionCall7Coll (fmgr.c:1293)
==00:00:00:04.044 873608==    by 0x6B6EEB: gincost_pattern
(selfuncs.c:7193)
==00:00:00:04.044 873608==    by 0x6B7132: gincost_opexpr
(selfuncs.c:7281)
==00:00:00:04.044 873608==    by 0x6BF1C3: gincostestimate
(selfuncs.c:7563)
==00:00:00:04.044 873608==    by 0x4AAD17: cost_index (costsize.c:588)
==00:00:00:04.044 873608==    by 0x4F640D: create_index_path
(pathnode.c:1028)
==00:00:00:04.044 873608==    by 0x4B6D0E: build_index_paths
(indxpath.c:1033)
==00:00:00:04.044 873608==    by 0x4B6F1D: get_index_paths
(indxpath.c:748)
==00:00:00:04.044 873608==  Address 0x108eab00 is 560 bytes inside a
recently re-allocated block of size 8,192 alloc'd
==00:00:00:04.044 873608==    at 0x4848899: malloc
(vg_replace_malloc.c:381)
==00:00:00:04.044 873608==    by 0x73A844: AllocSetContextCreateInternal
(aset.c:469)
==00:00:00:04.044 873608==    by 0x74C43F: tuplesort_begin_common
(tuplesort.c:868)
==00:00:00:04.044 873608==    by 0x752F75: tuplesort_begin_index_btree
(tuplesort.c:1217)
==00:00:00:04.044 873608==    by 0x26A40A: _bt_spools_heapscan
(nbtsort.c:477)
==00:00:00:04.044 873608==    by 0x26BE6B: btbuild (nbtsort.c:329)
==00:00:00:04.044 873608==    by 0x2E135F: index_build (index.c:3021)
==00:00:00:04.044 873608==    by 0x2E2F12: index_create (index.c:1252)
==00:00:00:04.044 873608==    by 0x30983D: create_toast_table
(toasting.c:324)
==00:00:00:04.044 873608==    by 0x309A2F: CheckAndCreateToastTable
(toasting.c:88)
==00:00:00:04.044 873608==    by 0x309A9D: NewRelationCreateToastTable
(toasting.c:75)
==00:00:00:04.044 873608==    by 0x5C942B: ProcessUtilitySlow
(utility.c:1199)
==00:00:00:04.044 873608==

The invalid access occurs in the line:
        while (j < arcsCount && arcs[j].sourceState == i)
here arcsCount == 1 even when arcs contains no elements, due to the
assignment above:
    arcsCount = (p2 - arcs) + 1;