[xslt] [PATCH 2/3] Fix NULL deref through valuePop retval: xsltKeyFunction

[Jan Pokorný <jpokorny redhat com>]

Test case (both template and the input):

<xsl:stylesheet version="1.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform";>
<!-- xsl:output method="text" indent="no"/ -->
<xsl:template match="*">
<xsl:value-of select="generate-id()=generate-id(key('none', $nonexistent))"/>
</xsl:template>
</xsl:stylesheet>

Before:
runtime error: file bug-generateid-deref.xsl line 5 element value-of
Variable 'nonexistent' has not been declared.
XPath error : Stack usage error
XPath error : Stack usage error
Segmentation fault (core dumped)

After (provided that original segfault in xsltGenerateIdFunction
was fixed first [patch preceding this one has been already provided
for that]):
runtime error: file bug-generateid-deref.xsl line 5 element value-of
Variable 'nonexistent' has not been declared.
XPath error : Stack usage errror
XPath error : Stack usage errror
runtime error: file bug-generateid-deref.xsl line 5 element value-of
generate-id() : invalid arg expecting a node-set
runtime error: file bug-generateid-deref.xsl line 5 element value-of
XPath evaluation returned no result.

The only drawback of the current patch is this sort of regression
in the test suite:

> ## Running general tests
> bug-180 result
> 4d3
> < xmlXPathCompiledEval: 1 objects left on the stack.
> ## Running general tests without dictionaries
> bug-180 result
> 4d3
> < xmlXPathCompiledEval: 1 objects left on the stack.

Signed-off-by: Jan Pokorný <jpokorny redhat com>
---
 libxslt/functions.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libxslt/functions.c b/libxslt/functions.c
index 38345c0..4a5475d 100644
--- a/libxslt/functions.c
+++ b/libxslt/functions.c
@@ -384,6 +384,8 @@ xsltKeyFunction(xmlXPathParserContextPtr ctxt, int nargs){
     * Get the key's name.
     */
     obj1 = valuePop(ctxt);
+    if (obj1 == NULL)
+        return;
 
     if ((obj2->type == XPATH_NODESET) || (obj2->type == XPATH_XSLT_TREE)) {
        int i;
-- 
1.8.1.4