bio: check for valid socket when closing

Fixes coverity 271258 Improper use of negative value (NEGATIVE_RETURNS)

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15943)

crypto/bio/bio_sock2.c

@@ -335,7 +335,7 @@ int BIO_accept_ex(int accept_sock, BIO_ADDR *addr_, int options)
  */
 int BIO_closesocket(int sock)
 {
-    if (closesocket(sock) < 0)
+    if (sock < 0 || closesocket(sock) < 0)
         return 0;
     return 1;
 }