Ensure that the string in SSH_AUTH_SOCK is not too large to fit in a Unix
sockaddr, and only copy the bytes of the supplied string.  The original code
goes on to copy bytes beyond the end of the string.

--- monotone-1.1/src/unix/ssh_agent_platform.cc.orig	2014-05-04 03:15:18.000000000 -0600
+++ monotone-1.1/src/unix/ssh_agent_platform.cc	2020-06-30 12:25:54.068518536 -0600
@@ -35,8 +35,9 @@ static int
 connect_to_agent()
 {
   const char *authsocket = getenv("SSH_AUTH_SOCK");
+  struct sockaddr_un addr;
 
-  if (!authsocket || !*authsocket)
+  if (!authsocket || !*authsocket || strlen(authsocket) >= sizeof(addr.sun_path))
     {
       L(FL("ssh_agent: no agent"));
       return -1;
@@ -57,9 +58,8 @@ connect_to_agent()
       return -1;
     }
 
-  struct sockaddr_un addr;
   addr.sun_family = AF_UNIX;
-  strncpy(addr.sun_path, authsocket, sizeof(addr.sun_path));
+  strcpy(addr.sun_path, authsocket);
 
   if (::connect(sock, (struct sockaddr *)&addr, sizeof addr))
     {