Clock freeze reaches unreachable code

[pfzetto]

Hi,
I'm using this crate with the STM32-IoT-Discovery-Kit.
When using the blinky example, I get a unreachable reached error on this line, because hclk < pclk1 in the examples.

Is this supposed to fail? If so, it should be a clear panic instead of a vague error that needs debugging.
Otherwise the unreachable call should be eliminated.