saa_fpwrite: initializing "len" should be part of the loop

"len" should properly be initialized on every turn of the loop.  It
can be initialized to any value >= blk_len that fits in a size_t.
(size_t)~0 would work except for any possible noncompliant C compilers
that have a signed size_t (illegal per C99 7.17.2).

nasmlib.c

@@ -727,8 +727,7 @@ void saa_fpwrite(struct SAA *s, FILE * fp)
     size_t len;
 
     saa_rewind(s);
-    len = s->datalen;
-    while ((data = saa_rbytes(s, &len)) != NULL)
+    while (len = s->datalen, (data = saa_rbytes(s, &len)) != NULL)
         fwrite(data, 1, len, fp);
 }