Fortify against missing purpose

(Humanity could use that)

bin/mkexec

     my $type = shift;
     my $purpose = shift;
     my $tmpl = "";
-    $tmpl .= $templates->{$type}->{$purpose} if exists $templates->{$type};
+    $tmpl .= $templates->{$type}->{$purpose}
+        if exists $templates->{$type}
+            and exists $templates->{$type}->{$purpose};
     return sprintf "#!%s\n%s\n", $bangs->{$type}, $tmpl;
 }