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.

include/mbedtls/net.h

@@ -45,6 +45,7 @@
 #define MBEDTLS_ERR_NET_CONN_RESET                        -0x0050  /**< Connection was reset by peer. */
 #define MBEDTLS_ERR_NET_UNKNOWN_HOST                      -0x0052  /**< Failed to get an IP address for the given hostname. */
 #define MBEDTLS_ERR_NET_BUFFER_TOO_SMALL                  -0x0043  /**< Buffer is too small to hold the data. */
+#define MBEDTLS_ERR_NET_INVALID_CONTEXT                   -0x0045  /**< The context is invalid, eg because it was free()ed. */
 
 #define MBEDTLS_NET_LISTEN_BACKLOG         10 /**< The backlog that listen() should use. */