Fix heap OOB read in zone_status()

Credit to Yaohui Zhu (朱耀辉) for finding this.

CNVD-C-2026-590208 applies.

Summary

zone_status() (src/dnssec.c:1867 in the 2.93 release; the same
construct is in current git master, 2.93-29-gad92083) computes
&name[name_start-2] in its backtrack loop without checking that
name_start >= 2. When name_start == 1 the loop starts at name[-1]
and dereferences it immediately; the guard (p != name) can never
fire for p < name, so the scan walks backwards through the heap
until it finds a '.' byte or hits an unmapped page. Under ASAN this
is a deterministic heap-buffer-overflow READ; on a plain build the
process dies with SIGSEGV.

A single, ordinary query plus one crafted upstream reply is enough.
My reproducer starts the ASAN binary with --dnssec and the root
trust anchor, with a mock upstream; on the first query the mock
answers with a captured crafted reply (a response carrying RRSIG(46),
NSEC(47) with an empty/root owner name, and NSEC3(50) records in the
authority section) and the process crashes on the first query.

CHANGELOG

@@ -8,6 +8,10 @@ version 2.94
 	constructed DHCPv6/RA - DHCPv6 and RA are inhibited by
 	--no-dhcpv4-interface when using constructed address ranges.
 	Thanks to David Nichols for discovery, diagnosis and patch.
+
+	Fix heap OOB read in zone_status. Thanks to Yaohui Zhu (朱耀辉)
+	for spotting and reporting this. A crafted reply can crash dnsmasq.
+	CNVD-C-2026-590208 applies.
 	
 	
 version 2.93

src/dnssec.c

@@ -1934,7 +1934,7 @@ static int zone_status(char *name, int class, char *keyname, time_t now)
 	    return STAT_INSECURE;
 	}
 
-      if (name_start == 0)
+      if (name_start <= 1)
 	break;
 
       for (p = &name[name_start-2]; (*p != '.') && (p != name); p--);