bug#60487: string-ref segfaults with n < 0 on Guile 3.0.8

[festerdam]

The following code results in a segmentation fault on Guile 
3.0.8-deb+3.0.8-2 (obtained from the Debian repositories):
    (string-ref "my string" -3)

gdb's backtrace is the following:

#0  0x00007ffff7f1bcc5 in ?? () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#1  0x00007ffff7f26c49 in scm_call_n ()
   from /lib/x86_64-linux-gnu/libguile-3.0.so.1
#2  0x00007ffff7e97b29 in scm_apply_0 ()
   from /lib/x86_64-linux-gnu/libguile-3.0.so.1
#3  0x00007ffff7f15966 in scm_throw ()
   from /lib/x86_64-linux-gnu/libguile-3.0.so.1
#4  0x00007ffff7f174e9 in scm_ithrow ()
   from /lib/x86_64-linux-gnu/libguile-3.0.so.1
#5  0x00007ffff7e94735 in scm_error_scm ()
   from /lib/x86_64-linux-gnu/libguile-3.0.so.1
#6  0x00007ffff7e94790 in scm_error ()
   from /lib/x86_64-linux-gnu/libguile-3.0.so.1
#7  0x00007ffff7ee19e7 in ?? () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#8  0x00007ffff7ee208b in scm_to_uint64 ()
   from /lib/x86_64-linux-gnu/libguile-3.0.so.1
#9  0x00007ffff7f1c5e4 in ?? () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#10 0x00007ffff7f26c49 in scm_call_n ()
   from /lib/x86_64-linux-gnu/libguile-3.0.so.1
#11 0x00007ffff7e93a97 in scm_primitive_eval ()
   from /lib/x86_64-linux-gnu/libguile-3.0.so.1
#12 0x00007ffff7e99a86 in scm_eval ()
   from /lib/x86_64-linux-gnu/libguile-3.0.so.1
#13 0x00007ffff7ef91c6 in scm_shell () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#14 0x00007ffff7ea865c in ?? () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#15 0x00007ffff7e91f6a in ?? () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#16 0x00007ffff7f194e8 in ?? () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#17 0x00007ffff7f26c49 in scm_call_n () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#18 0x00007ffff7e936ea in scm_call_2 () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#19 0x00007ffff7f42292 in ?? () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#20 0x00007ffff7f0ff4f in scm_c_catch () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#21 0x00007ffff7e942e6 in scm_c_with_continuation_barrier () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#22 0x00007ffff7f14b89 in ?? () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#23 0x00007ffff7c190e7 in GC_call_with_stack_base () from 
/lib/x86_64-linux-gnu/libgc.so.1
#24 0x00007ffff7f0fe68 in scm_with_guile () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#25 0x00007ffff7eb1185 in scm_boot_guile () from 
/lib/x86_64-linux-gnu/libguile-3.0.so.1
#26 0x000055555555510f in ?? ()
#27 0x00007ffff7c9918a in __libc_start_call_main 
(main=main@entry=0x5555555550b0, argc=argc@entry=1, 
argv=argv@entry=0x7fffffffe0b8) at 
../sysdeps/nptl/libc_start_call_main.h:58
#28 0x00007ffff7c99245 in __libc_start_main_impl (main=0x5555555550b0, 
argc=1, argv=0x7fffffffe0b8, init=<optimized out>, fini=<optimized out>, 
rtld_fini=<optimized out>, stack_end=0x7fffffffe0a8) at 
../csu/libc-start.c:381
#29 0x00005555555551aa in ?? ()