rts: Ensure that new capability count is > 0
rts: Ensure that new capability count is > 0

The Haskell wrapper already checks this but we should also check it in the RTS
to catch non-Haskell callers. See #13832.

shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc