BUILD: stconn: fix build warning at -O3 about possible null sc

BUILD: stconn: fix build warning at -O3 about possible null sc

gcc-6.x and 7.x emit build warnings about sc possibly being null upon
return from sc_detach_endp(). This actually is not the case and the
compiler is a little bit overzealous there, but there exists code
paths that can make this analysis non-trivial so let's at least add
a similar BUG_ON() to let both the compiler and the deverloper know
this doesn't happen.

This should be backported to 2.6.