Input: fix infinite loop

Right shift on a signed value can extend the sign bit, which would make
this function do an infinite loop if passed a negative value. Use an
unsigned instead to get the behavior we want.

Fixes #15957, but this means we're using undefined data from the
settings file. I assumed it would have the unused button values set to 0
but this isn't apparently the case. Preetpal is already working on that
as we noticed the problem when testing her patch for adding a 6th
button.

Change-Id: I62fdc778ff3b9da92f3aa5570163dc60baf8cf2c
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2560
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

src/preferences/input/MouseView.cpp

@@ -64,7 +64,7 @@ getButtonOffsets(int32 type)
 
 
 static uint32
-getMappingNumber(int32 mapping)
+getMappingNumber(uint32 mapping)
 {
 	if (mapping == 0)
 		return 0;