cdump: fix uninitialized warning with optimization

cdump: fix uninitialized warning with optimization

/home/rusty/devel/cvs/ccan/ccan/cdump/cdump.c: In function ‘get_type’:
/home/rusty/devel/cvs/ccan/ccan/strmap/strmap.h:88:39: warning: ‘m’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  tcon_cast((map), canary, strmap_get_(&(map)->raw, (member)))
                                       ^
/home/rusty/devel/cvs/ccan/ccan/cdump/cdump.c:216:20: note: ‘m’ was declared here
  cdump_map_t *m;
               ^

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>