wallet-test: fix segfault due to uninitialized block

The variable `block` (instace of `struct block`) is
allocated on the stack without being initialized, i.e. its
member `prev` points to nowhere. This causes a segmentation
fault on my machine on the binding of "prev_hash" on running
`wallet_block_add`, as the following core-dump analysis
shows:

    $ egdb ./wallet/test/run-wallet ./run-wallet.core
    [...]
    Core was generated by `run-wallet'.
    Program terminated with signal SIGSEGV, Segmentation fault.
    ---Type <return> to continue, or q <return> to quit---
    #0  0x000008f67a04b660 in memcpy (dst0=<optimized out>, src0=0x100007f8c, length=32) at /usr/src/lib/libc/string/memcpy.c:97
    97                      TLOOP1(*dst++ = *src++);
    (gdb) bt
    #0  0x000008f67a04b660 in memcpy (dst0=<optimized out>, src0=0x100007f8c, length=32) at /usr/src/lib/libc/string/memcpy.c:97
    ElementsProject#1  0x000008f73e838f60 in sqlite3VdbeMemSetStr () from /usr/local/lib/libsqlite3.so.37.12
    ElementsProject#2  0x000008f73e83cb11 in bindText () from /usr/local/lib/libsqlite3.so.37.12
    ElementsProject#3  0x000008f44bc91345 in db_sqlite3_query (stmt=0x8f6845bf028) at wallet/db_sqlite3.c:77
    ElementsProject#4  0x000008f44bc91122 in db_sqlite3_exec (stmt=0x8f6845bf028) at wallet/db_sqlite3.c:110
    ElementsProject#5  0x000008f44bcbb3b2 in db_exec_prepared_v2 (stmt=0x8f6845bf028) at ./wallet/db.c:2055
    ElementsProject#6  0x000008f44bcc6890 in wallet_block_add (w=0x8f688b5bba8, b=0x7f7ffffca788) at ./wallet/wallet.c:3556
    ElementsProject#7  0x000008f44bce2607 in test_wallet_outputs (ld=0x8f6a35a7828, ctx=0x8f6a35c0268) at wallet/test/run-wallet.c:1104
    ElementsProject#8  0x000008f44bcddec0 in main (argc=1, argv=0x7f7ffffcaaf8) at wallet/test/run-wallet.c:1930

Fix by explicitely setting the whole structure to zero.

[ Rebuilt generated files, too --RR ]

wallet/test/run-wallet.c

@@ -1099,6 +1099,7 @@ static bool test_wallet_outputs(struct lightningd *ld, const tal_t *ctx)
 	u32 *blockheight = tal(w, u32);
 	*blockheight = 100;
 	/* We gotta add a block to the database though */
+	memset(&block, 0, sizeof(block));
 	block.height = 100;
 	memset(&block.blkid, 2, sizeof(block.blkid));
 	wallet_block_add(w, &block);