This patch corrects an issue in the compiler whereby calling
Quote_Argument with an argument that is of size 1 may lead to a
CONSTRAINT_ERROR raised at runtime due to an undersized buffer.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* libgnat/s-os_lib.adb (Quote_Argument): Modify the result
	buffer size calculation to handle the case where Arg'Length is
	1.