bugfix for test_bson_reserve_buffer_errors

The earlier fix for a bug in allocating bson_t near the max size revealed a flawed test that was only succeeding because of the allocation bug.
This fixes the test to identify both sides of the boundary: we fail to allocate a bson_t that's one byte too large, and on 64-bit platforms we allocate a max-sized bson_t.

Author: Micah Scott

src/libbson/tests/test-bson.c

@@ -1886,7 +1886,11 @@ test_bson_reserve_buffer_errors (void)
    uint32_t len_le;
 
    /* too big */
-   ASSERT (!bson_reserve_buffer (&bson, (uint32_t) (INT32_MAX - bson.len - 1)));
+   ASSERT (!bson_reserve_buffer (&bson, (uint32_t) (BSON_MAX_SIZE - bson.len + 1u)));
+   /* exactly the maximum size */
+#if BSON_WORD_SIZE > 32
+   ASSERT (bson_reserve_buffer (&bson, (uint32_t) (BSON_MAX_SIZE - bson.len)));
+#endif
 
    /* make a static bson, it refuses bson_reserve_buffer since it's read-only */
    bson_destroy (&bson);