loader: don't call realloc(non_null, 0) when no symbols are present

According to C99, realloc(non_null, 0) != free(non_null), that's why
it is forbidden in QEMU.

When there are no symbols, nsyms equals to 0. Free the syms structure
and set it to NULL instead of reallocating it with a size of 0.

This fixes -kernel with stripped kernels.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Acked-by: Michael S. Tsirkin <mst@redhat.com>