Fix uninitialized variable in an edge case

If `context_buf = mbedtls_calloc( 1, buf_len )` failed,
`context_buf_len` was not initialized. Noticed by
`gcc -Os -Werror=maybe-uninitialized`.

This was only a problem in ssl_server2 (a test program), only with
MBEDTLS_SSL_CONTEXT_SERIALIZATION enabled.

programs/ssl/ssl_server2.c

@@ -1820,7 +1820,7 @@ int main( int argc, char *argv[] )
 #endif
 #if defined(MBEDTLS_SSL_CONTEXT_SERIALIZATION)
     unsigned char *context_buf = NULL;
-    size_t context_buf_len;
+    size_t context_buf_len = 0;
 #endif
 
     int i;