ice40: incorrectly invoking set_io aborts

[whitequark]

E.g.:

# top.pcf
set_io io[0]

terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check: __n (which is 2) >= this->size() (which is 2)
build_top.sh: line 5: 25970 Aborted                 nextpnr-ice40 --up5k --package sg48 --pcf top.pcf --json top.json --asc top.txt

Backtrace:

#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1  0x00007ffff60ba2f1 in __GI_abort () at abort.c:79
#2  0x00007ffff647c943 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3  0x00007ffff6482896 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#4  0x00007ffff64828d1 in std::terminate() () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#5  0x00007ffff6482b04 in __cxa_throw () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#6  0x00007ffff647e851 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#7  0x00000000006a1927 in std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >::_M_range_check (this=<optimized out>, __n=<optimized out>)
    at /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/stl_vector.h:960
#8  std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >::at (this=<optimized out>, __n=<optimized out>)
    at /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/stl_vector.h:981
#9  nextpnr_ice40::apply_pcf (ctx=0xb9ee490, filename=<incomplete type>, in=...) at /home/whitequark/Projects/nextpnr/ice40/pcf.cc:53
#10 0x000000000068d1a6 in Ice40CommandHandler::customAfterLoad (this=<optimized out>, ctx=0xb9ee490)
    at /home/whitequark/Projects/nextpnr/ice40/main.cc:84
#11 0x00000000005d9d1c in nextpnr_ice40::CommandHandler::executeMain (this=0x7fffffffd768, ctx=...)
    at /home/whitequark/Projects/nextpnr/common/command.cc:228
#12 0x00000000005dc0e1 in nextpnr_ice40::CommandHandler::exec (this=0x7fffffffd768) at /home/whitequark/Projects/nextpnr/common/command.cc:303
#13 0x000000000068e5b7 in main (argc=<optimized out>, argv=<optimized out>) at /home/whitequark/Projects/nextpnr/ice40/main.cc:161

[whitequark]

Thanks!