[Ada] Fix 0-sized secondary stack allocations

[Ada] Fix 0-sized secondary stack allocations

[Pierre-Marie de Rodat]

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The Has_Enough_Free_Memory was not correctly reporting a completely full
chunk in the case of a 0-sized allocation.

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

gcc/ada/

	* libgnat/s-secsta.adb (Has_Enough_Free_Memory): Check for full
	chunk before computing the available size.

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diff --git a/gcc/ada/libgnat/s-secsta.adb b/gcc/ada/libgnat/s-secsta.adb
--- a/gcc/ada/libgnat/s-secsta.adb
+++ b/gcc/ada/libgnat/s-secsta.adb
@@ -506,12 +506,17 @@ package body System.Secondary_Stack is
       Mem_Size : Memory_Size) return Boolean
    is
    begin
+      --  First check if the chunk is full (Byte is > Memory'Last in that
+      --  case), then check there is enough free memory.
+
       --  Byte - 1 denotes the last occupied byte. Subtracting that byte from
       --  the memory capacity of the chunk yields the size of the free memory
       --  within the chunk. The chunk can fit the request as long as the free
       --  memory is as big as the request.
 
-      return Chunk.Size - (Byte - 1) >= Mem_Size;
+      return Chunk.Memory'Last >= Byte
+        and then Chunk.Size - (Byte - 1) >= Mem_Size;
+
    end Has_Enough_Free_Memory;
 
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