XS: Ensure created Perl strings are nul-terminated

Most of these are binary strings and shouldn't be interpreted as C
strings in the first place, but better be safe in case they are, anyway.
The lack of nul-termination of FU::Pg `$hex` strings was more likely to
be problematic.

c/fdpass.c

@@ -71,6 +71,7 @@ static int fufdpass_recv(pTHX_ I32 ax, int socket, size_t len) {
     }
 
     SvCUR_set(buf, r);
+    SvPVX(buf)[r] = 0;
     ST(1) = buf;
     return 2;
 }