handle the case where 0 or >1 allocs are enabled

In particular this fixes `cargo build --all-features`.

src/alloc/hardened.rs

@@ -1,5 +1,3 @@
-#![cfg(all(not(target_env = "msvc"), feature = "hardened_malloc", target_os = "linux", not(feature = "jemalloc")))]
-
 #[global_allocator]
 static HMALLOC: hardened_malloc_rs::HardenedMalloc = hardened_malloc_rs::HardenedMalloc;