Fix memory leak when encoding bare bignums

This fixes a leak when encoding a bare bignum. Technically it would be
possible to hit this memory leak randomly with bignums in objects but
the chances are highly unlikely.

Thanks to @miriampena for the issue.

Fixes #69

c_src/encoder.c

@@ -205,17 +205,22 @@ enc_unknown(Encoder* e, ERL_NIF_TERM value)
     // a bignum might produce when encoded.
     e->iosize += e->i + 16;
 
-    // Reinitialize our binary for the next buffer.
-    e->curr = bin;
-    if(!enif_alloc_binary(BIN_INC_SIZE, e->curr)) {
-        return 0;
-    }
+    // Reinitialize our binary for the next buffer if we
+    // used any data in the buffer. If we haven't used any
+    // bytes in the buffer then we can safely reuse it
+    // for anything following the unknown value.
+    if(e->i > 0) {
+        e->curr = bin;
+        if(!enif_alloc_binary(BIN_INC_SIZE, e->curr)) {
+            return 0;
+        }
 
-    memset(e->curr->data, 0, e->curr->size);
+        memset(e->curr->data, 0, e->curr->size);
 
-    e->p = (char*) e->curr->data;
-    e->u = (unsigned char*) e->curr->data;
-    e->i = 0;
+        e->p = (char*) e->curr->data;
+        e->u = (unsigned char*) e->curr->data;
+        e->i = 0;        
+    }
 
     return 1;
 }

test/jiffy_14_bignum_memory_leak.erl

@@ -0,0 +1,29 @@
+% This file is part of Jiffy released under the MIT license.
+% See the LICENSE file for more information.
+
+-module(jiffy_14_bignum_memory_leak).
+
+-include_lib("eunit/include/eunit.hrl").
+
+
+bignum_encoding_leak_test_() ->
+    run_gc(),
+    Before = erlang:memory(binary),
+    encode_bignums(1000000),
+    run_gc(),
+    After = erlang:memory(binary),
+    ?_assert(After - Before < 100000).
+
+
+run_gc() ->
+    [erlang:garbage_collect(Pid) || Pid <- erlang:processes()].
+
+
+encode_bignums(N) ->
+    {_, Ref} = spawn_monitor(fun() ->
+        [jiffy:encode(1072502107250210725021072502) || _ <- lists:seq(1, N)]
+    end),
+    receive
+        {'DOWN', Ref, process, _, _} ->
+            ok
+    end.