Check for max length before derefing by length.

Coverity CID 135025 (#1 of 1): Out-of-bounds read (OVERRUN)
29. overrun-local: Overrunning array addr.sun_path of 108 bytes at byte offset 108 using index addr_len (which evaluates to 108).
 864                for (addr_len = 0; addr.sun_path[addr_len]
 28. incr: Incrementing addr_len. The value of addr_len may now be up to 108.
 865                     && addr_len < maxlen; addr_len++);

Reported upstream as

https://rt.cpan.org/Ticket/Display.html?id=111707

cpan/Socket/Socket.xs

@@ -861,8 +861,8 @@ unpack_sockaddr_un(sun_sv)
 #   else
 		const int maxlen = (int)sizeof(addr.sun_path);
 #   endif
-		for (addr_len = 0; addr.sun_path[addr_len]
-		     && addr_len < maxlen; addr_len++);
+		for (addr_len = 0; addr_len < maxlen
+		     && addr.sun_path[addr_len]; addr_len++);
 	}
 
 	ST(0) = sv_2mortal(newSVpvn(addr.sun_path, addr_len));