Add error handling on realpath() call.

(Without this, it happened for me that realpath() failed returning
undef for the default vardir. This in turn caused mysql-test-run.pl to
delete the source mysql-test/ directory.)

Backport from 10.1, it's not nice to get one's source directory nuked
by a rouge mysql-test-run.

mysql-test/mysql-test-run.pl

@@ -1502,7 +1502,12 @@ sub command_line_setup {
   {
     $default_vardir= "$glob_mysql_test_dir/var";
   }
-  $default_vardir = realpath $default_vardir unless IS_WINDOWS;
+  unless (IS_WINDOWS) {
+    my $realpath = realpath($default_vardir);
+    die "realpath('$default_vardir') failed: $!\n"
+      unless defined($realpath) && $realpath ne '';
+    $default_vardir = $realpath;
+  }
 
   if ( ! $opt_vardir )
   {