rust/protover: return C-allocated string in protover_all_supported()

The result of CString::into_raw() is not safe to free
with free() except under finicky and fragile circumstances
that we definitely don't meet right now.

This was missed in be583a34a3.

changes/bug27740

@@ -0,0 +1,4 @@
+  o Minor bugfixes (rust):
+    - Return a string that can be safely freed by C code, not one created by
+      the rust allocator, in protover_all_supported(). Fixes bug 27740; bugfix
+      on 0.3.3.1-alpha.

src/rust/protover/ffi.rs

@@ -71,12 +71,7 @@ pub extern "C" fn protover_all_supported(
         if missing_out.is_null() {
             return 0;
         }
-        let c_unsupported: CString = match CString::new(unsupported.to_string()) {
+        let ptr = allocate_and_copy_string(&unsupported.to_string());
-            Ok(n) => n,
-            Err(_) => return 1,
-        };
-
-        let ptr = c_unsupported.into_raw();
         unsafe { *missing_out = ptr };
 
         return 0;