[CM] s7_load_c_string

[bil at ccrma.Stanford.EDU]

I'd expect a free cell complaint if it was the GC.  Didn't
Christos run into a similar problem in early July (unbound
variable from s7_load_c_string)? -- I think it turned out
to be a missing null at the end of the string (and maybe
off-by-1 length?).  I believe a file might not end in
0, so gzip/unzip might produce an unterminated string.
s7_load_c_string should check for the 0.