Bug 1810351

Description of problem:
arachne-pnr crashes on icestorm demo

Version-Release number of selected component (if applicable):
arachne-pnr-0.1-0.7.20190729gitc40fb22.fc31.x86_64

How reproducible:
Consistently

Steps to Reproduce:
Test example from http://www.clifford.at/icestorm/

test$ cat example.v
// example.v
module top (input a, b, output y);
  assign y = a & b;
endmodule

test$ cat example.pcf 
# example.pcf
set_io a 1
set_io b 10
set_io y 11
test$ rpm -q yosys
yosys-0.9-1.fc31.x86_64
test$ rpm -q arachne-pnr
arachne-pnr-0.1-0.7.20190729gitc40fb22.fc31.x86_64
test$ yosys -p 'synth_ice40 -top top -blif example.blif' example.v
...
test$ arachne-pnr -d 1k -o example.asc -p example.pcf example.blif

Actual results:
test$ arachne-pnr -d 1k -o example.asc -p example.pcf example.blif
seed: 1
device: 1k
read_chipdb +/share/arachne-pnr/chipdb-1k.bin...
  supported packages: cb121, cb132, cb81, cm121, cm36, cm49, cm81, qn84, swg16tr, tq144, vq100
read_blif example.blif...
prune...
read_pcf example.pcf...
instantiate_io...
pack...

After packing:
IOs          3 / 96
GBs          0 / 8
  GB_IOs     0 / 8
LCs          1 / 1280
  DFF        0
  CARRY      0
  CARRY, DFF 0
  DFF PASS   0
  CARRY PASS 0
BRAMs        0 / 16
WARMBOOTs    0 / 1
PLLs         0 / 1

place_constraints...
promote_globals...
  promoted 0 nets
  0 globals
realize_constants...
place...
  initial wire length = 35
  at iteration #50: temp = 11.8101, wire length = 22
  at iteration #100: temp = 2.68105, wire length = 11
  at iteration #150: temp = 0.00167343, wire length = 7
  final wire length = 7

After placement:
PIOs       3 / 96
PLBs       1 / 160
BRAMs      0 / 16

  place time 0.00s
route...
/usr/include/c++/9/bits/stl_vector.h:1042: std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::operator[](std::vector<_Tp, _Alloc>::size_type) [with _Tp = std::pair<int, int>; _Alloc = std::allocator<std::pair<int, int> >; std::vector<_Tp, _Alloc>::reference = std::pair<int, int>&; std::vector<_Tp, _Alloc>::size_type = long unsigned int]: Assertion '__builtin_expect(__n < this->size(), true)' failed.
Aborted (core dumped)



Expected results:
Successfully create example.asc file

Additional info:
Backtrace:
(gdb) where
#0  0x00007ffff7aa3625 in raise () from /lib64/libc.so.6
#1  0x00007ffff7a8c8d9 in abort () from /lib64/libc.so.6
#2  0x0000000000432908 in std::__replacement_assert (
    __file=__file@entry=0x493078 "/usr/include/c++/9/bits/stl_vector.h", 
    __line=__line@entry=1042, 
    __function=__function@entry=0x498858 "std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::operator[](std::vector<_Tp, _Alloc>::size_type) [with _Tp = std::pair<int, int>; _Alloc = std::allocator<std::pair<int, int> >; std::vecto"..., 
    __condition=__condition@entry=0x492f18 "__builtin_expect(__n < this->size(), true)") at /usr/include/c++/9/x86_64-redhat-linux/bits/c++config.h:2533
#3  0x0000000000456f31 in std::vector<std::pair<int, int>, std::allocator<std::pair<int, int> > >::operator[] (__n=<optimized out>, this=0x7fffffffb670)
    at /usr/include/c++/9/new:174
#4  PriorityQ<std::pair<int, int>, Comp>::push (x={...}, this=0x7fffffffb668)
    at src/priorityq.hh:46
#5  Router::visit (this=this@entry=0x7fffffffb3b0, cn=1520) at src/route.cc:601
#6  0x0000000000457108 in Router::start (this=this@entry=0x7fffffffb3b0, 
    net=net@entry=0) at src/route.cc:541
#7  0x0000000000460319 in Router::route (this=0x7fffffffb3b0)
    at src/route.cc:793
#8  0x0000000000461f54 in route (ds=..., max_passes=max_passes@entry=200)
    at src/route.cc:980
#9  0x000000000041039e in main (argc=<optimized out>, argv=<optimized out>)
    at src/arachne-pnr.cc:637