S390: For zero, let is_power_of_two() return false

S390: For zero, let is_power_of_two() return false

This fixes a minor issue with the helper function is_power_of_two(),
which returned non-zero ("true") if the argument was zero.  This led
to a wrong decision when passing a zero-sized struct in an inferior
function call.

gdb/ChangeLog:

	* s390-linux-tdep.c (is_power_of_two): Add comment.  Return
	  false if the argument is zero.