Fix invalid memcpy(dst, src) calls where dst == src

The convert_generic_inline method was not correctly handling the case
where both the conversion methods were passthrough and the last store
is therefore not needed and may trigger an invalid memcpy call.

Change-Id: Ic88780f50e1ff9dedc04b8ff1ab3527dd0c8150c
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>