ESP: Add a workaround to a "use-of-uninitialized-value"

ESP: Add a workaround to a "use-of-uninitialized-value"

Found with clang, CFLAGS=-fsanitize=memory.

Fix GitHub issues #848 and #849.

The problem is that for some unknown reason the pt buffer is not
initialized after EVP_DecryptUpdate() call, no error, in:

print-esp.c:260:        if (!EVP_DecryptUpdate(ctx, pt, &len, ct, ctlen)) {