Fix another ancient bug in parsing of BRE-mode regular expressions.

While poking at the regex code, I happened to notice that the bug
squashed in commit afcc8772e had a sibling: next() failed to return
a specific value associated with the '}' token for a "\{m,n\}"
quantifier when parsing in basic RE mode.  Again, this could result
in treating the quantifier as non-greedy, which it never should be in
basic mode.  For that to happen, the last character before "\}" that
sets "nextvalue" would have to set it to zero, or it'd have to have
accidentally been zero from the start.  The failure can be provoked
repeatably with, for example, a bound ending in digit "0".

Like the previous patch, back-patch all the way.

src/backend/regex/regc_lex.c

@@ -389,7 +389,7 @@ next(struct vars *v)
 					{
 						v->now++;
 						INTOCON(L_BRE);
-						RET('}');
+						RETV('}', 1);
 					}
 					else
 						FAILW(REG_BADBR);