PerlIO::scalar: tests for trailing null

using Eric Brine’s function.

ext/PerlIO-scalar/t/scalar.t

@@ -16,7 +16,7 @@ use Fcntl qw(SEEK_SET SEEK_CUR SEEK_END); # Not 0, 1, 2 everywhere.
 
 $| = 1;
 
-use Test::More tests => 76;
+use Test::More tests => 79;
 
 my $fh;
 my $var = "aaa\n";
@@ -318,6 +318,7 @@ EOF
     pass 'seeking on a glob copy from the end';
 }
 
+# [perl #108398]
 sub has_trailing_nul(\$) {
    my ($ref) = @_;
    my $sv = B::svref_2object($ref);
@@ -331,3 +332,25 @@ sub has_trailing_nul(\$) {
    my $trailing = unpack 'P', pack 'J', $pv_addr+$cur;
    return $trailing eq "\0";
 }
+SKIP: {
+    if ($Config::Config{'extensions'} !~ m!\bPerlIO/scalar\b!) {
+	skip "no B", 1;
+    }
+    require B;
+
+    open my $fh, ">", \my $memfile or die $!;
+
+    print $fh "abc";
+    ok has_trailing_nul $memfile,
+	 'write appends trailing null when growing string';
+
+    seek $fh, 0,SEEK_SET;
+    print $fh "abc";
+    ok has_trailing_nul $memfile,
+	 'write appends trailing null when not growing string';
+
+    seek $fh, 200, SEEK_SET;
+    print $fh "abc";
+    ok has_trailing_nul $memfile,
+	 'write appends null when growing string after seek past end';
+}