c1c crashes with error "free(): invalid next size (fast): 0x00007fffd618e7d0"

I tried compiling the test.c1 program from the repository and c1c crashed with the following backtrace:

*** Error in `./c1c': free(): invalid next size (fast): 0x00007fffd618e7d0 ***
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x70bfb)[0x7f943d8d0bfb]
/lib/x86_64-linux-gnu/libc.so.6(+0x76fc6)[0x7f943d8d6fc6]
/lib/x86_64-linux-gnu/libc.so.6(+0x7780e)[0x7f943d8d780e]
./c1c(+0x122e)[0x7f943e00122e]
./c1c(+0xfc6)[0x7f943e000fc6]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf1)[0x7f943d8802e1]
./c1c(+0xaba)[0x7f943e000aba]
======= Memory map: ========
7f9438000000-7f9438021000 rw-p 00000000 00:00 0
7f9438021000-7f943c000000 ---p 00000000 00:00 0
7f943d640000-7f943d656000 r-xp 00000000 00:00 534852             /lib/x86_64-linux-gnu/libgcc_s.so.1
7f943d656000-7f943d657000 ---p 00016000 00:00 534852             /lib/x86_64-linux-gnu/libgcc_s.so.1
7f943d657000-7f943d855000 ---p 00000017 00:00 534852             /lib/x86_64-linux-gnu/libgcc_s.so.1
7f943d855000-7f943d856000 r--p 00015000 00:00 534852             /lib/x86_64-linux-gnu/libgcc_s.so.1
7f943d856000-7f943d857000 rw-p 00016000 00:00 534852             /lib/x86_64-linux-gnu/libgcc_s.so.1
7f943d860000-7f943d9f5000 r-xp 00000000 00:00 534827             /lib/x86_64-linux-gnu/libc-2.24.so
7f943d9f5000-7f943d9fd000 ---p 00195000 00:00 534827             /lib/x86_64-linux-gnu/libc-2.24.so
7f943d9fd000-7f943dbf5000 ---p 0000019d 00:00 534827             /lib/x86_64-linux-gnu/libc-2.24.so
7f943dbf5000-7f943dbf9000 r--p 00195000 00:00 534827             /lib/x86_64-linux-gnu/libc-2.24.so
7f943dbf9000-7f943dbfb000 rw-p 00199000 00:00 534827             /lib/x86_64-linux-gnu/libc-2.24.so
7f943dbfb000-7f943dbff000 rw-p 00000000 00:00 0
7f943dc00000-7f943dc22000 r-xp 00000000 00:00 534805             /lib/x86_64-linux-gnu/ld-2.24.so
7f943dc22000-7f943dc23000 r-xp 00022000 00:00 534805             /lib/x86_64-linux-gnu/ld-2.24.so
7f943de23000-7f943de24000 r--p 00023000 00:00 534805             /lib/x86_64-linux-gnu/ld-2.24.so
7f943de24000-7f943de25000 rw-p 00024000 00:00 534805             /lib/x86_64-linux-gnu/ld-2.24.so
7f943de25000-7f943de26000 rw-p 00000000 00:00 0
7f943df00000-7f943df02000 rw-p 00000000 00:00 0
7f943df10000-7f943df11000 rw-p 00000000 00:00 0
7f943df20000-7f943df23000 rw-p 00000000 00:00 0
7f943e000000-7f943e001000 r-xp 00000000 00:00 1446759            /home/clemens/c1c/c1c
7f943e001000-7f943e002000 r-xp 00001000 00:00 1446759            /home/clemens/c1c/c1c
7f943e201000-7f943e202000 r--p 00001000 00:00 1446759            /home/clemens/c1c/c1c
7f943e202000-7f943e203000 rw-p 00002000 00:00 1446759            /home/clemens/c1c/c1c
7fffd618e000-7fffd61af000 rw-p 00000000 00:00 0                  [heap]
7fffdccbd000-7fffdd4bd000 rw-p 00000000 00:00 0                  [stack]
7fffdda28000-7fffdda29000 r-xp 00000000 00:00 0                  [vdso]
Aborted (core dumped)

I am using GCC 6.3.0 on WSL (Debian).

Reproduce the crash by compiling test.c1 with c1c in the reported GCC 6.3.0 Debian-on-WSL environment. Start from the provided backtrace and use a debugger or memory checker to locate the invalid free. Done means test.c1 compiles without the allocator abort and the regression is covered.