fileno might return negative.

(Coverity CID 104853)

Also rewrite the fstat call to test for zero (success) explicitly,
instead of the sneaky bang-negation.

sv.c

@@ -8306,7 +8306,8 @@ Perl_sv_gets(pTHX_ SV *const sv, PerlIO *const fp, I32 append)
 	   the size we read (e.g. CRLF or a gzip layer).
 	 */
 	Stat_t st;
-	if (!PerlLIO_fstat(PerlIO_fileno(fp), &st) && S_ISREG(st.st_mode))  {
+        int fd = PerlIO_fileno(fp);
+	if (fd >= 0 && (PerlLIO_fstat(fd, &st) == 0) && S_ISREG(st.st_mode))  {
 	    const Off_t offset = PerlIO_tell(fp);
 	    if (offset != (Off_t) -1 && st.st_size + append > offset) {
 #ifdef PERL_NEW_COPY_ON_WRITE