[Ada] Fix problematic underflow for Float_Type'Value

We need a couple of guards for boundary conditions in the support code.

gcc/ada/

	* libgnat/s-dourea.adb ("/"): Add guard for zero and infinite
	divisor.
	* libgnat/s-valuer.adb (Scan_Raw_Real): Add guard for very large
	exponent values.

gcc/ada/libgnat/s-dourea.adb

@@ -178,6 +178,12 @@ package body System.Double_Real is
       P, R   : Double_T;
 
    begin
+      if Is_Infinity (B) or else Is_Zero (B) then
+         return (A.Hi / B, 0.0);
+      end if;
+      pragma Annotate (CodePeer, Intentional, "test always false",
+                       "code deals with infinity");
+
       Q1 := A.Hi / B;
 
       --  Compute R = A - B * Q1
@@ -196,6 +202,12 @@ package body System.Double_Real is
       R, S       : Double_T;
 
    begin
+      if Is_Infinity (B.Hi) or else Is_Zero (B.Hi) then
+         return (A.Hi / B.Hi, 0.0);
+      end if;
+      pragma Annotate (CodePeer, Intentional, "test always false",
+                       "code deals with infinity");
+
       Q1 := A.Hi / B.Hi;
       R := A - B * Q1;
 

gcc/ada/libgnat/s-valuer.adb

@@ -645,7 +645,14 @@ package body System.Value_R is
 
       Ptr.all := Index;
       Scan_Exponent (Str, Ptr, Max, Expon, Real => True);
-      Scale := Scale + Expon;
+
+      --  Handle very large exponents like Scan_Exponent
+
+      if Expon < Integer'First / 10 or else Expon > Integer'Last / 10 then
+         Scale := Expon;
+      else
+         Scale := Scale + Expon;
+      end if;
 
       --  Here is where we check for a bad based number