Configuration/15-android.conf: slightly move NDK canonisation

This allows the original path to be displayed when it's shown
to be invalid, so the user can relate without question.
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6925)

Configurations/15-android.conf

@@ -24,8 +24,8 @@
 
             my $ndk = $ENV{ANDROID_NDK};
             die "\$ANDROID_NDK is not defined"  if (!$ndk);
-            $ndk = canonpath($ndk);
             die "\$ANDROID_NDK=$ndk is invalid" if (!-d "$ndk/platforms");
+            $ndk = canonpath($ndk);
 
             my $ndkver = undef;