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/pgtypes.c

@@ -166,6 +166,7 @@ RECVFN(hex) {
         *out++ = PL_hexdigit[(in[i] >> 4) & 0x0f];
         *out++ = PL_hexdigit[in[i] & 0x0f];
     }
+    *out = 0;
     SvCUR_set(r, len * 2);
     return r;
 }