BUG/MINOR: stconn: don't wake up an applet waiting on buffer allocation

BUG/MINOR: stconn: don't wake up an applet waiting on buffer allocation

Since the extension of the buffers API to applets in 3.0-dev, an applet
may find itself unable to allocate a buffer, and will block respectively
on APPCTX_FL_OUTBLK_ALLOC or APPCTX_FL_INBLK_ALLOC depending on the
direction. However the code in sc_applet_process() doesn't consider this
situation when deciding to wake up an applet, so when the condition
arises, the applet keeps ringing and is killed by the loop detector.

The fix is trivial and simply consists in checking for the flags above.
No backport is needed since this is new in 3.0.