Avoid careless dereferences outside the domain name buffer.
Part of this is
https://github.com/nmap/nmap/commit/3adaa69cb211b00f9bfc66263a56cbd87cc9e521

Index: nmap_dns.cc
--- nmap_dns.cc.orig
+++ nmap_dns.cc
@@ -1352,7 +1352,7 @@ bool DNS::Factory::ptrToIp(const std::string &ptr, soc
   memset(&ip, 0, sizeof(sockaddr_storage));
 
   // Check whether the name ends with the IPv4 PTR domain
-  if (NULL != (p = strcasestr(cptr + ptr.length() + 1 - sizeof(C_IPV4_PTR_DOMAIN), C_IPV4_PTR_DOMAIN)))
+  if (ptr.length() >= sizeof(C_IPV4_PTR_DOMAIN) - 1 && NULL != (p = strcasestr(cptr + ptr.length() + 1 - sizeof(C_IPV4_PTR_DOMAIN), C_IPV4_PTR_DOMAIN)))
   {
     struct sockaddr_in *ip4 = (struct sockaddr_in *)&ip;
     u8 place_value[] = {1, 10, 100};
@@ -1361,7 +1361,7 @@ bool DNS::Factory::ptrToIp(const std::string &ptr, soc
     size_t i = 0;
 
     p--;
-    while (i < sizeof(ip4->sin_addr.s_addr))
+    while (p >= cptr && i < sizeof(ip4->sin_addr.s_addr))
     {
       if (*p == '.')
       {
@@ -1387,14 +1387,14 @@ bool DNS::Factory::ptrToIp(const std::string &ptr, soc
     ip.ss_family = AF_INET;
   }
   // If not, check IPv6
-  else if (NULL != (p = strcasestr(cptr + ptr.length() + 1 - sizeof(C_IPV6_PTR_DOMAIN), C_IPV6_PTR_DOMAIN)))
+  else if (ptr.length() >= sizeof(C_IPV6_PTR_DOMAIN) - 1 && NULL != (p = strcasestr(cptr + ptr.length() + 1 - sizeof(C_IPV6_PTR_DOMAIN), C_IPV6_PTR_DOMAIN)))
   {
     struct sockaddr_in6 *ip6 = (struct sockaddr_in6 *)&ip;
     u8 alt = 0;
     size_t i=0;
 
     p--;
-    while (i < sizeof(ip6->sin6_addr.s6_addr))
+    while (p >= cptr && i < sizeof(ip6->sin6_addr.s6_addr))
     {
       if (*p == '.')
       {