Die if tor_vasprintf fails in connection_printf_to_buf

tor_asprintf already asserts if it fails.

changes/bug3428b

@@ -0,0 +1,9 @@
+  o Minor bugfixes:
+    - Abort if tor_vasprintf fails in connection_printf_to_buf (a
+      utility function used in the control-port code).  This shouldn't
+      ever happen unless Tor is completely out of memory, but if it
+      had happened and Tor somehow recovered from it, Tor could have
+      sent a log message to a control port in the middle of a reply to
+      a controller command.  Fixes part of bug 3428.
+
+

src/or/control.c

@@ -503,8 +503,8 @@ connection_printf_to_buf(control_connection_t *conn, const char *format, ...)
   va_end(ap);
 
   if (len < 0) {
-    log_warn(LD_BUG, "Unable to format string for controller.");
-    return;
+    log_err(LD_BUG, "Unable to format string for controller.");
+    tor_assert(0);
   }
 
   connection_write_to_buf(buf, (size_t)len, TO_CONN(conn));