bug#30869: regexp-substitute/global does not like #\nul in string

[Matt Wette]

version = 2.2.3

I think this is a bug: regexp-substitute/global rejects strings with #\nul:
scheme@(guile-user)> (regexp-substitute/global #f "b.d" "ab\x00de" 'pre 
'post)
ERROR: In procedure regexp-exec:
string contains #\nul character: "ab\x00de"