exec: Fix file_ram_alloc() error API violations

exec: Fix file_ram_alloc() error API violations

When os_mem_prealloc() fails, file_ram_alloc() calls qemu_ram_munmap()
and returns null.  Except it doesn't when its @errp argument is null,
because it checks for failure with (errp && *errp).  Introduced in
commit 056b68af77 "fix qemu exit on memory hotplug when allocation
fails at prealloc time".

No caller actually passes null.

Fix anyway: splice in a local Error *err, and error_propagate().

Cc: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20191204093625.14836-6-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>