efi: fix memory leak in krealloc failure handling

In the previous code, there was a memory leak issue where the
previously allocated memory was not freed upon a failed krealloc
operation. This patch addresses the problem by releasing the old memory
before setting the pointer to NULL in case of a krealloc failure. This
ensures that memory is properly managed and avoids potential memory
leaks.

Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>