Fix ordering of free()ing of internal structures in ssl_server2

If `MBEDTLS_MEMORY_BUFFER_ALLOC_C` is configured and Mbed TLS'
custom buffer allocator is used for calloc() and free(), the
read buffer used by the server example application is allocated
from the buffer allocator, but freed after the buffer allocator
has been destroyed. If memory backtracing is enabled, this leaves
a memory leak in the backtracing structure allocated for the buffer,
as found by valgrind.

Fixes #2069.

programs/ssl/ssl_server2.c

@@ -3146,6 +3146,8 @@ exit:
     mbedtls_ssl_cookie_free( &cookie_ctx );
 #endif
 
+    mbedtls_free( buf );
+
 #if defined(MBEDTLS_MEMORY_BUFFER_ALLOC_C)
 #if defined(MBEDTLS_MEMORY_DEBUG)
     mbedtls_memory_buffer_alloc_status();
@@ -3153,7 +3155,6 @@ exit:
     mbedtls_memory_buffer_alloc_free();
 #endif
 
-    mbedtls_free( buf );
     mbedtls_printf( " done.\n" );
 
 #if defined(_WIN32)