Use abort() instead of assert(false) for things that should never happen

assert() is compiled out for release builds, but we don't want to
continue running in these impossible situations.

This also resolves the "implicit fallthrough" warnings for the asserts
in switch cases.

src/nvim/eval/executor.c

@@ -118,7 +118,7 @@ int eexe_mod_op(typval_T *const tv1, const typval_T *const tv2,
         return OK;
       }
       case VAR_UNKNOWN: {
-        assert(false);
+        abort();
       }
     }
   }