hw/core/loader-fit: Fix missing ERRP_GUARD() for error_prepend()

hw/core/loader-fit: Fix missing ERRP_GUARD() for error_prepend()

As the comment in qapi/error, passing @errp to error_prepend() requires
ERRP_GUARD():

* = Why, when and how to use ERRP_GUARD() =
*
* Without ERRP_GUARD(), use of the @errp parameter is restricted:
...
* - It should not be passed to error_prepend(), error_vprepend() or
*   error_append_hint(), because that doesn't work with &error_fatal.
* ERRP_GUARD() lifts these restrictions.
*
* To use ERRP_GUARD(), add it right at the beginning of the function.
* @errp can then be used without worrying about the argument being
* NULL or &error_fatal.

ERRP_GUARD() could avoid the case when @errp is &error_fatal, the user
can't see this additional information, because exit() happens in
error_setg earlier than information is added [1].

In hw/core/loader-fit.c, there are 2 functions passing @errp to
error_prepend() without ERRP_GUARD():
 - fit_load_kernel()
 - fit_load_fdt()

Their @errp parameters are both the pointers of the local @err virable
in load_fit().

Though they don't cause the issue like [1] said, to follow the
requirement of @errp, add missing ERRP_GUARD() at their beginning.

[1]: Issue description in the commit message of commit ae7c80a7bd73
     ("error: New macro ERRP_GUARD()").

Cc: Paul Burton <paulburton@kernel.org>
Cc: Aleksandar Rikalo <aleksandar.rikalo@syrmia.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Message-ID: <20240311033822.3142585-15-zhao1.liu@linux.intel.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>