Remove CString drop test.

The test relies on the undefined behavior, and so may fail in some
circumstances. This can be worked around by stubbing a memory allocator
in the test, but it is a bit of work, and LLVM could still theoretically
eliminate the write of the zero byte in release mode (which is
intended).

So let's just remove the test and mark the function as inline. It
shouldn't be optimized away when inlined into the debug build of user's
code.

Author: matklad

src/libstd/ffi/c_str.rs

@@ -314,9 +314,11 @@ impl CString {
 }
 
 // Turns this `CString` into an empty string to prevent
-// memory unsafe code from working by accident.
+// memory unsafe code from working by accident. Inline
+// to prevent LLVM from optimizing it away in debug builds.
 #[stable(feature = "cstring_drop", since = "1.13.0")]
 impl Drop for CString {
+    #[inline]
     fn drop(&mut self) {
         unsafe { *self.inner.get_unchecked_mut(0) = 0; }
     }