Fix segfault on overloaded arithmetic assignment

Fix segfault on overloaded arithmetic assignment

Consider an arithmetic assignment operation of the form

  $left += $right

A segfault was occuring in the case where $right is an overloaded object
but $left is not; and where $right does not override "+=" but does
provide a 'nomethod' override.  Internally, Perl_amagic_call was
attempting to clone $left as if it were an overloaded object, causing the
segfault.  This commit fixes the segfault by only cloning the left
operand when the left operand is the overloaded one.