make uninitialized global and local variables cause exception.

tests/testjet.jet

@@ -1,4 +1,4 @@
-#!/home/barant/Development/JetCore/jet-2.0
+#!../jet-2.0
 <jet cgi="true" name1="localname" filterblanklines="true" trimlines="true">
  <header name="Content-Type" value="text/html" />
  <comment>This is a comment and should not show up in the output.</comment>

tests/testvar.jet

@@ -19,4 +19,5 @@ $[var$[i$[letterx]]$[i$[letterx]]]
 $[ix]
 <set name="ix" expr="$[ix]+1" />
 $[ix]
+#[iy]
 </jet>