riscv64/uart/sifive: If we get an invalid clock, skip setting baud

* Prevents us breaking a potentially working serial uart.

Change-Id: I57bed42cfa571d3d961ee07f380c96b62c7f34d6
Reviewed-on: https://review.haiku-os.org/c/haiku/+/8798
Reviewed-by: waddlesplash <[email protected]>

src/system/kernel/arch/riscv64/arch_uart_sifive.cpp

@@ -37,7 +37,13 @@ ArchUARTSifive::Init()
 void
 ArchUARTSifive::InitPort(uint32 baud)
 {
-	uint64 quotient = (Clock() + baud - 1) / baud;
+	int64 clock = Clock();
+
+	// if we get an invalid clock for some reason, just accept the currently set baud.
+	if (clock < 0)
+		return;
+
+	uint64 quotient = (clock + baud - 1) / baud;
 
 	if (quotient == 0)
 		Regs()->div = 0;