r-value matrix subscripted causing crashes

[111116]

>>> float2x2(1,2,3,4)
float2x2([1,2], [3,4])
>>> float2x2(1,2,3,4)[0]
float2(9.731018e-35,1e-45)

[111116]

fixed 9154455

return_value_policy::reference_internal Indicates that the lifetime of the return value is tied to the lifetime of a parent object, namely the implicit this, or self argument of the called method or property. Internally, this policy works just like return_value_policy::reference but additionally applies a keep_alive<0, 1> call policy (described in the next section) that prevents the parent object from being garbage collected as long as the return value is referenced by Python. This is the default policy for property getters created via def_property, def_readwrite, etc.