Use memcpy instead of strncpy in s48_enter_substring.

S48 1.0.1 does this and it's faster
.

c/external.c

@@ -1114,7 +1114,7 @@ s48_value
 s48_enter_substring(char *str, int length)
 {
   s48_value obj = s48_allocate_stob(S48_STOBTYPE_STRING, length + 1);
-  strncpy(S48_UNSAFE_EXTRACT_STRING(obj), str, length);
+  memcpy(S48_UNSAFE_EXTRACT_STRING(obj), str, length);
   *(S48_UNSAFE_EXTRACT_STRING(obj) + length) = '\0';
   return obj;
 }