Tighten up pointer handling after ssh_pkt_getstring.

ssh_pkt_getstring can return (NULL,0) if the input packet is too short
to contain a valid string.

In quite a few places we were passing the returned pointer,length pair
to a printf function with "%.*s" type format, which seems in practice
to have not been dereferencing the pointer but the C standard doesn't
actually guarantee that. In one place we were doing the same job by
hand with memcpy, and apparently that _can_ dereference the pointer in
practice (so a server could have caused a NULL-dereference crash by
sending an appropriately malformed "x11" type channel open request).
And also I spotted a logging call in the "forwarded-tcpip" channel
open handler which had forgotten the field width completely, so it was
erroneously relying on the string happening to be NUL-terminated in
the received packet.

I've tightened all of this up in general by normalising (NULL,0) to
("",0) before calling printf("%.*s"), and replacing the two even more
broken cases with the corrected version of that same idiom.

misc.h

@@ -169,4 +169,9 @@ void debug_memdump(void *buf, int len, int L);
   (cp)[0] = (unsigned char)((value) >> 8), \
   (cp)[1] = (unsigned char)(value) )
 
+/* Replace NULL with the empty string, permitting an idiom in which we
+ * get a string (pointer,length) pair that might be NULL,0 and can
+ * then safely say things like printf("%.*s", length, NULLTOEMPTY(ptr)) */
+#define NULLTOEMPTY(s) ((s)?(s):"")
+
 #endif