[Ada] Fix logic in C_Select under mingw

[Pierre-Marie de Rodat]

GNAT started generated a new legitimate warning on g-socthi__mingw.adb:
g-socthi.adb:395:19: warning: condition is always True

After further investigation, it appears that Original_WFS was
unconditionally dereferencing Writefds without first checking against null.

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

gcc/ada/

        * libgnat/g-socthi__mingw.adb (C_Select): Fix logic in code and
        make it explicit that we are checking against null values before
        dereferencing them.
diff --git a/gcc/ada/libgnat/g-socthi__mingw.adb b/gcc/ada/libgnat/g-socthi__mingw.adb
--- a/gcc/ada/libgnat/g-socthi__mingw.adb
+++ b/gcc/ada/libgnat/g-socthi__mingw.adb
@@ -375,13 +375,10 @@ package body GNAT.Sockets.Thin is
       Exceptfds : access Fd_Set;
       Timeout   : Timeval_Access) return C.int
    is
-      pragma Warnings (Off, Exceptfds);
-
-      Original_WFS : aliased constant Fd_Set := Writefds.all;
-
-      Res  : C.int;
-      S    : aliased C.int;
-      Last : aliased C.int;
+      Original_WFS : aliased Fd_Set;
+      Res          : C.int;
+      S            : aliased C.int;
+      Last         : aliased C.int;
 
    begin
       --  Asynchronous connection failures are notified in the exception fd
@@ -392,7 +389,8 @@ package body GNAT.Sockets.Thin is
       --  present in the initial write fd set, then move the socket from the
       --  exception fd set to the write fd set.
 
-      if Writefds /= No_Fd_Set_Access then
+      if Writefds /= null then
+         Original_WFS := Writefds.all;
 
          --  Add any socket present in write fd set into exception fd set
 
@@ -411,7 +409,7 @@ package body GNAT.Sockets.Thin is
 
       Res := Standard_Select (Nfds, Readfds, Writefds, Exceptfds, Timeout);
 
-      if Exceptfds /= No_Fd_Set_Access then
+      if Exceptfds /= null then
          declare
             EFSC    : aliased Fd_Set := Exceptfds.all;
             Flag    : constant C.int := SOSC.MSG_PEEK + SOSC.MSG_OOB;
@@ -448,8 +446,8 @@ package body GNAT.Sockets.Thin is
                   --  exception fd set back to write fd set. Otherwise, ignore
                   --  this event since the user is not watching for it.
 
-                  if Writefds /= No_Fd_Set_Access
-                    and then (Is_Socket_In_Set (Original_WFS'Access, S) /= 0)
+                  if Writefds /= null
+                    and then Is_Socket_In_Set (Original_WFS'Access, S) /= 0
                   then
                      Insert_Socket_In_Set (Writefds, S);
                   end if;
@@ -457,6 +455,7 @@ package body GNAT.Sockets.Thin is
             end loop;
          end;
       end if;
+
       return Res;
    end C_Select;