Fix potential use-after-free

Clippy pointed out this potential use-after-free of a CString buffer. I
don't think in this case it caused an issue because the temporary
CString _should_ outlive the function call (at least, that's how C++
works...), however it's refactor-brittle, so best to fix.

https://github.com/Manishearth/rust-clippy/wiki#temporary_cstring_as_ptr

src/cursor.rs

@@ -290,7 +290,6 @@ mod test {
     use flags::*;
     use super::*;
     use test_utils::*;
-    use transaction::*;
 
     #[test]
     fn test_get() {

src/environment.rs

@@ -1,5 +1,5 @@
 use libc::{c_uint, size_t};
-use std::ffi::CString;
+use std::ffi::{CString, NulError};
 #[cfg(unix)]
 use std::os::unix::ffi::OsStrExt;
 #[cfg(windows)]
@@ -207,10 +207,11 @@ impl EnvironmentBuilder {
                 lmdb_try_with_cleanup!(ffi::mdb_env_set_mapsize(env, map_size),
                                        ffi::mdb_env_close(env))
             }
-            lmdb_try_with_cleanup!(ffi::mdb_env_open(env,
-                                                     CString::new(path.as_os_str().as_bytes()).unwrap().as_ptr(),
-                                                     self.flags.bits(),
-                                                     mode),
+            let path = match CString::new(path.as_os_str().as_bytes()) {
+                Ok(path) => path,
+                Err(NulError { .. }) => return Err(::Error::Invalid),
+            };
+            lmdb_try_with_cleanup!(ffi::mdb_env_open(env, path.as_ptr(), self.flags.bits(), mode),
                                    ffi::mdb_env_close(env));
         }
         Ok(Environment { env: env, dbi_open_mutex: Mutex::new(()) })

src/error.rs

@@ -145,5 +145,4 @@ mod test {
         assert_eq!("MDB_NOTFOUND: No matching key/data pair found",
                    Error::NotFound.description());
     }
-
 }