Assume pointer is not null in GlobalAlloc::dealloc

Signed-off-by: David McNeil <mcneil.david2@gmail.com>

src/lib.rs

@@ -122,6 +122,7 @@ unsafe impl GlobalAlloc for Jemalloc {
 
     #[inline]
     unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+        assume!(!ptr.is_null())
         assume!(layout.size() != 0);
         let flags = layout_to_flags(layout.align(), layout.size());
         ffi::sdallocx(ptr as *mut c_void, layout.size(), flags)