Avoid infinite recursion after failed param evaluation

Always mark variables as computed even if evaluation fails. This
avoids infinite recursion and a stack overflow if a parameter whose
evaluation failed is looked up again.

Found with afl-fuzz.

libxslt/variables.c

@@ -1803,8 +1803,7 @@ xsltBuildVariable(xsltTransformContextPtr ctxt,
     elem->tree = tree;
     elem->value = xsltEvalVariable(ctxt, elem,
 	(xsltStylePreCompPtr) comp);
-    if (elem->value != NULL)
-	elem->computed = 1;
+    elem->computed = 1;
     return(elem);
 }
 

tests/docs/bug-194.xml

+<doc/>

tests/general/bug-194.err

+runtime error
+Variable 'p' has not been declared.
+xmlXPathCompiledEval: evaluation failed
+runtime error: file ./bug-194.xsl line 4 element param
+Failed to evaluate the expression of variable 'p'.
+xmlXPathCompiledEval: evaluation failed
+runtime error: file ./bug-194.xsl line 5 element param
+Failed to evaluate the expression of variable 'p2'.
+no result for ./../docs/bug-194.xml

tests/general/bug-194.xsl

+<x:stylesheet xmlns:x="http://www.w3.org/1999/XSL/Transform" version="1.0">
+
+<x:template match="/">
+    <x:param name="p" select="$p"/>
+    <x:param name="p2" select="$p"/>
+</x:template>
+
+</x:stylesheet>