target/i386: avoid theoretical leak on MCE injection
g_strdup_printf is used twice to write to the same variable, which
can theoretically cause a leak. In practice, it is extremely
unlikely that a guest is seeing a recursive MCE and has disabled
CR4.MCE between the first and the second error, but we can fix it
and we can also make a slight improvement on the logic: CR4.MCE=0
causes a triple fault even for a non-recursive machine check, so
let's place its test first.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>