Fix https://github.com/rprichard/winpty/issues/67

The str array need an extra element for the NUL terminator.  The previous
code didn't suffer from memory-unsafety, but winpty_snprintf truncated the
last character of the marker, and trying to write NUL to the WinXP console
didn't work.  (I suspect WinXP turned the NUL into a space, so
findSyncMarker couldn't find it later.)

src/agent/Agent.cc

@@ -839,7 +839,7 @@ void Agent::syncMarkerText(CHAR_INFO (&output)[SYNC_MARKER_LEN])
 {
     // XXX: The marker text generated here could easily collide with ordinary
     // console output.  Does it make sense to try to avoid the collision?
-    char str[SYNC_MARKER_LEN];
+    char str[SYNC_MARKER_LEN + 1];
     winpty_snprintf(str, "S*Y*N*C*%08x", m_syncCounter);
     for (int i = 0; i < SYNC_MARKER_LEN; ++i) {
         output[i].Char.UnicodeChar = str[i];