copy_bytes() recover function

The version of copy_bytes() in util/recover.c had fallen behind
the internal version in files.c which had received some
analyzer changes in 4bc5e26082 from
December 2022.

This synchronizes them, and addresses a couple of other static
analysis recommendations.

src/files.c

@@ -4456,9 +4456,10 @@ boolean
 copy_bytes(int ifd, int ofd)
 {
     char buf[BUFSIZ];
-    int nfrom, nto = 0;
+    int nfrom, nto;
 
     do {
+        nto = 0;
         nfrom = read(ifd, buf, BUFSIZ);
         /* read can return -1 */
         if (nfrom >= 0 && nfrom <= BUFSIZ)