erts: Fix sigq buffer removal
erts: Fix sigq buffer removal

The `nonmsg_slots` of the current buffer was not cleared after
flushing when the buffer was removed, breaking the flag handling
in `erts_proc_sig_fetch`.

jj1bdx/otp,dgud/otp,g-andrade/otp,erlang/otp,potatosalad/otp,dgud/otp,potatosalad/otp,bjorng/otp,emacsmirror/erlang,isvilen/otp,dumbbell/otp,jj1bdx/otp,erlang/otp,vinoski/otp,vinoski/otp,vinoski/otp,bjorng/otp,vinoski/otp,erlang/otp,isvilen/otp,potatosalad/otp,bjorng/otp,rlipscombe/otp,bjorng/otp,dumbbell/otp,emacsmirror/erlang,g-andrade/otp,potatosalad/otp,potatosalad/otp,jj1bdx/otp,emacsmirror/erlang,mikpe/otp,dumbbell/otp,rlipscombe/otp,dgud/otp,vinoski/otp,g-andrade/otp,vinoski/otp,mikpe/otp,isvilen/otp,vinoski/otp,isvilen/otp,bjorng/otp,emacsmirror/erlang,jj1bdx/otp,dumbbell/otp,rlipscombe/otp,emacsmirror/erlang,dumbbell/otp,mikpe/otp,isvilen/otp,jj1bdx/otp,mikpe/otp,dumbbell/otp,potatosalad/otp,dumbbell/otp,dgud/otp,erlang/otp,mikpe/otp,dgud/otp,rlipscombe/otp,rlipscombe/otp,erlang/otp,dgud/otp,mikpe/otp,erlang/otp,dumbbell/otp,g-andrade/otp,potatosalad/otp,bjorng/otp,erlang/otp,g-andrade/otp,isvilen/otp,vinoski/otp,rlipscombe/otp,isvilen/otp,potatosalad/otp,g-andrade/otp,potatosalad/otp,bjorng/otp,mikpe/otp,erlang/otp,emacsmirror/erlang,erlang/otp,jj1bdx/otp,mikpe/otp,rlipscombe/otp,g-andrade/otp,g-andrade/otp,bjorng/otp,potatosalad/otp,jj1bdx/otp,isvilen/otp,mikpe/otp,dgud/otp,bjorng/otp,isvilen/otp,emacsmirror/erlang,jj1bdx/otp,rlipscombe/otp,emacsmirror/erlang,dgud/otp,rlipscombe/otp,erlang/otp,rlipscombe/otp,emacsmirror/erlang,g-andrade/otp,jj1bdx/otp,g-andrade/otp,dgud/otp,emacsmirror/erlang,dumbbell/otp,bjorng/otp,vinoski/otp,dumbbell/otp,jj1bdx/otp,vinoski/otp,isvilen/otp,dgud/otp,mikpe/otp