This fixes a valgrind read past buffer error... I was seeing when
testing t.archos/Linux-x86_64/dbg_maps
```
Invalid read of size 8
==11424== at 0x4C2F79E: memcpy@@GLIBC_2.14 (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==11424== by 0x693C702: r_reg_set_bytes (arena.c:118)
[...]
==11424== Address 0xa144430 is 0 bytes after a block of size 304 alloc'd
==11424== at 0x4C2CC70: calloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==11424== by 0x5D49EE6: r_debug_reg_sync (dreg.c:60)
[...]
```
---
v2:
* don't forget spaces around '+' (pancake)
