Add guards for closed socket in net.c
This is particularly problematic when calling FD_SET( -1, ... ), but let's
check it in all functions.
This was introduced with the new API and the fact the net_free() now sets the
internal fd to -1 in order to mark it as closed: now using this information.
diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index cb5a697..1ffb876 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -62,7 +62,7 @@
* DES 1 0x0032-0x0032
* CTR_DBRG 4 0x0034-0x003A
* ENTROPY 3 0x003C-0x0040 0x003D-0x003F
- * NET 9 0x0042-0x0052 0x0043-0x0043
+ * NET 11 0x0042-0x0052 0x0043-0x0045
* ASN1 7 0x0060-0x006C
* PBKDF2 1 0x007C-0x007C
* HMAC_DRBG 4 0x0003-0x0009