Avoid passing NULL argument to memcpy

bitcoin/pullpush.c

@@ -1,5 +1,6 @@
 #include "pullpush.h"
 #include "varint.h"
+#include <assert.h>
 #include <ccan/endian/endian.h>
 #include <ccan/mem/mem.h>
 #include <ccan/tal/tal.h>
@@ -58,6 +59,7 @@ const u8 *pull(const u8 **cursor, size_t *max, void *copy, size_t n)
 	}
 	*cursor += n;
 	*max -= n;
+	assert(p);
 	if (copy)
 		memcpy(copy, p, n);
 	return memcheck(p, n);