New zero_ptr lint.

This fixes #1047. I did not bother to add a full suggestion,
because replacing with `std::ptr::null()` may still lead to
inference failures.

clippy_lints/src/lib.rs

@@ -450,6 +450,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
         misc_early::REDUNDANT_CLOSURE_CALL,
         misc_early::UNNEEDED_FIELD_PATTERN,
         misc_early::ZERO_PREFIXED_LITERAL,
+        misc_early::ZERO_PTR,
         mut_reference::UNNECESSARY_MUT_PASSED,
         mutex_atomic::MUTEX_ATOMIC,
         needless_bool::BOOL_COMPARISON,