Fix broken error path on open failure in PROXY_FUZZ.

We have to use the file name we just failed to open to format an error
message _before_ freeing it, not after. If that use-after-free managed
not to cause a crash, we'd also leak the file descriptor 'outfd'.

Both spotted by Coverity (which is probably the first thing in years
to look seriously at any of the code designed for Ben's AFL exercise).

unix/uxproxy.c

@@ -88,8 +88,11 @@ Socket *platform_new_connection(SockAddr *addr, const char *hostname,
 	}
 	infd = open(cmd, O_RDONLY);
 	if (infd == -1) {
+            Socket *toret = new_error_socket_fmt(
+                plug, "%s: %s", cmd, strerror(errno));
 	    sfree(cmd);
-	    return new_error_socket_fmt(plug, "%s: %s", cmd, strerror(errno));
+            close(outfd);
+	    return toret;
 	}
 	sfree(cmd);
 	inerrfd = -1;