Don't use str::as_mut_ptr in tests since it's not stable in 1.34.2

tests/compilation.rs

@@ -74,5 +74,6 @@ fn test_mut_pointer() {
     }
 
     let mut s = String::new();
-    thing(s.as_mut_ptr());
+    // `as_mut_ptr` is not stable in 1.34.2
+    thing(s.as_mut_str() as *mut str as *mut u8);
 }