efi_loader: correctly check if the HTTP protocol is found

efi_loader: correctly check if the HTTP protocol is found

In function efi_http_service_binding_destroy_child() phandler is created as
as a local variable. If efi_search_protocol() fails, phandler will hold a
random value from the stack. Even it is not zero, we must not use it.

If efi_search_protocol() succeeds, the pointer has already be dereferenced,
so checking against NULL makes not sense here.

If ChildHandle is not a valid UEFI handle, we must return
EFI_INVALID_PARAMETER.

Use a single location for EFI_EXIT().

Addresses-Coverity-ID: CID 531974 (Unchecked return value)
Fixes: 5753dc3f6572 ("efi_loader: Prevent dereference of uninitialised variable")
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>