This was found because it means we have a non-zero feerate without
filling in the history of that feerate:
==15895== Conditional jump or move depends on uninitialised value(s)
==15895== at 0x408699: feerate_max (chaintopology.c:828)
==15895== by 0x41BE49: peer_start_openingd (opening_control.c:733)
==15895== by 0x425FE9: peer_connected (peer_control.c:515)
==15895== by 0x40CB8F: connectd_msg (connect_control.c:304)
==15895== by 0x42DB4E: sd_msg_read (subd.c:475)
==15895== by 0x42D499: read_fds (subd.c:302)
==15895== by 0x46EB18: next_plan (io.c:59)
==15895== by 0x46F5E9: do_plan (io.c:387)
==15895== by 0x46F627: io_ready (io.c:397)
==15895== by 0x471187: io_loop (poll.c:310)
==15895== by 0x41683D: main (lightningd.c:732)
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>