ada: Refactor code to remove GNATcheck violation

Rewrite for loop containing an exit (which violates GNATcheck
rule Exits_From_Conditional_Loops), to use a while loop
which contains the exit criteria in its condition.
Also, move special case of first time through loop, to come
before loop.

gcc/ada/

	* libgnat/s-imagef.adb (Set_Image_Fixed): Refactor loop.