nostrdb/blocks: add ndb_blocks_free

In some situations we will need to have owned note blocks. For
example, when we try to fetch note blocks from the database and it's not
there yet. We will need to parse the content on the spot and return an
owned copy, since it will not be immediately available in the database.

Add a new flag field to note blocks that lets us know if it's owned by
malloc or nostrdb.

We the add a free function that checks this flag and frees the object if
its set. If it is not set then it doesn nothing because it likely came
from the database.

Signed-off-by: William Casarin <jb55@jb55.com>

nostrdb/src/block.h

@@ -9,11 +9,14 @@
 #include "nostrdb.h"
 #include <inttypes.h>
 
+#define NDB_BLOCK_FLAG_OWNED 1
+
 #pragma pack(push, 1)
 
 struct ndb_blocks {
 	unsigned char version;
-	unsigned char padding[3];
+	unsigned char flags;
+	unsigned char padding[2];
 
 	uint32_t words;
 	uint32_t num_blocks;