kernel_loader: check phdr memory size addition
kernel_loader: check phdr memory size addition

The mem_offset + phdr.memsz addition is using untrusted input
(phdr.memsz) and can overflow; add an explicit check to avoid panics on
invalid values.

BUG=None
TEST=/usr/libexec/fuzzers/crosvm_zimage_fuzzer in cros_fuzz shell

Change-Id: Ie6f7f27bd00958ff85201cecaa75ce2b19779b8b
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1674664
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm