SecurityPkg/RngDxe: add rng test

Check whenever RngLib actually returns random numbers, only return
a non-zero number of Algorithms if that is the case.

This has the effect that RndDxe loads and installs EFI_RNG_PROTOCOL
only in case it can actually deliver random numbers.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

SecurityPkg/RandomNumberGenerator/RngDxe/Rand/RngDxe.c

@@ -23,6 +23,7 @@
 
 #include <Library/BaseLib.h>
 #include <Library/BaseMemoryLib.h>
+#include <Library/RngLib.h>
 
 #include "RngDxeInternals.h"
 
@@ -43,7 +44,12 @@ GetAvailableAlgorithms (
   VOID
   )
 {
+  UINT64  RngTest;
+
+  if (GetRandomNumber64 (&RngTest)) {
     mAvailableAlgoArrayCount = RNG_ALGORITHM_COUNT;
+  }
+
   return EFI_SUCCESS;
 }