ShellPkg: Fix check on OldArgv in UpdateArgcArgv()

The UpdateArgcArgv() function documentation says "If OldArgv or OldArgc
is NULL then that value is not returned."

However, only OldArgc was checked for NULL, probably because of
copy-pasto. In case OldArgc was non-NULL, but OldArgv was null, it could
cause a segmentation fault.

Check OldArgv is not NULL before dereferencing the value.

Signed-off-by: Tormod Volden <[email protected]>

ShellPkg/Application/Shell/ShellParametersProtocol.c

@@ -1470,7 +1470,7 @@ UpdateArgcArgv (
     *OldArgc = ShellParameters->Argc;
   }
 
-  if (OldArgc != NULL) {
+  if (OldArgv != NULL) {
     *OldArgv = ShellParameters->Argv;
   }