BUG #16814: Invalid memory access on regexp_match with .* and BRE

The following bug has been logged on the website:

Bug reference: 16814
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 13.1
Operating system: Ubuntu 20.04
Description:

When executing the following regexp call:
select regexp_match('abc', '.*', 'b');
valgrind detects an error:
==00:00:00:46.767 138746== Conditional jump or move depends on uninitialised
value(s)
==00:00:00:46.767 138746== at 0x4657A9: parseqatom (regcomp.c:990)
==00:00:00:46.767 138746== by 0x465CBD: parsebranch (regcomp.c:753)
==00:00:00:46.767 138746== by 0x465E84: parse (regcomp.c:683)
==00:00:00:46.767 138746== by 0x467F24: pg_regcomp (regcomp.c:404)
==00:00:00:46.767 138746== by 0x57D100: RE_compile_and_cache
(regexp.c:185)
==00:00:00:46.767 138746== by 0x57D3D9: setup_regexp_matches
(regexp.c:1114)
==00:00:00:46.767 138746== by 0x57DF86: regexp_match (regexp.c:985)
==00:00:00:46.767 138746== by 0x36839A: ExecInterpExpr
(execExprInterp.c:699)
==00:00:00:46.767 138746== by 0x3657C9: ExecInterpExprStillValid
(execExprInterp.c:1802)
==00:00:00:46.767 138746== by 0x42A172: ExecEvalExprSwitchContext
(executor.h:316)
==00:00:00:46.767 138746== by 0x42A172: evaluate_expr (clauses.c:4809)
==00:00:00:46.767 138746== by 0x42A34B: evaluate_function
(clauses.c:4339)
==00:00:00:46.767 138746== by 0x42C1ED: simplify_function
(clauses.c:3969)

(This was discovered on the back of the new test module test_regex with the
slightly modified 30.4:
select * from test_regex('.*b', 'aab', 'b');
)