MSVC: Avoid warning when testing a TAP suite without PROVE_FLAGS.

Commit 7be5d8df1f74b78620167d3abf32ee607e728919 surfaced the logic
error, which had no functional implications, by adding "use warnings".
The buildfarm always customizes PROVE_FLAGS, so the warning did not
appear there.  Back-patch to 9.5 (all supported versions).

src/tools/msvc/vcregress.pl

@@ -198,7 +198,7 @@ sub tap_check
 	  unless $config->{tap_tests};
 
 	my @flags;
-	foreach my $arg (0 .. scalar(@_))
+	foreach my $arg (0 .. scalar(@_) - 1)
 	{
 		next unless $_[$arg] =~ /^PROVE_FLAGS=(.*)/;
 		@flags = split(/\s+/, $1);