This patch taken directly from upstream:

http://git.gnome.org/browse/libxml2/commit/?id=1d4526f6f4ec8d18c40e2a09b387652a6c1aa2cd

--- libxml2-2.7.6/xpath.c	Fri Dec  9 06:34:26 2011

+++ libxml2-2.7.6/new.xpath.c	Fri Dec  9 06:43:12 2011

@@ -2485,6 +2485,7 @@

                                              sizeof(ctxt->valueTab[0]));

         if (tmp == NULL) {

             xmlGenericError(xmlGenericErrorContext, "realloc failed !\n");

+	    ctxt->error = XPATH_MEMORY_ERROR;

             return (0);

         }

         ctxt->valueMax *= 2;

@@ -9327,6 +9328,7 @@

 		if ( (ch & 0xc0) != 0xc0 ) {

 		    xmlGenericError(xmlGenericErrorContext,

 			"xmlXPathTranslateFunction: Invalid UTF8 string\n");

+		    /* not asserting an XPath error is probably better */

 		    break;

 		}

 		/* then skip over remaining bytes for this char */

@@ -9334,6 +9336,7 @@

 		    if ( (*cptr++ & 0xc0) != 0x80 ) {

 			xmlGenericError(xmlGenericErrorContext,

 			    "xmlXPathTranslateFunction: Invalid UTF8 string\n");

+			/* not asserting an XPath error is probably better */

 			break;

 		    }

 		if (ch & 0x80) /* must have had error encountered */

@@ -13384,6 +13387,7 @@

                         xmlGenericError(xmlGenericErrorContext,

             "xmlXPathCompOpEval: variable %s bound to undefined prefix %s\n",

                                     (char *) op->value4, (char *)op->value5);

+			ctxt->error = XPATH_UNDEF_PREFIX_ERROR;

                         return (total);

                     }

 		    val = xmlXPathVariableLookupNS(ctxt->context,

@@ -13438,6 +13442,7 @@

             "xmlXPathCompOpEval: function %s bound to undefined prefix %s\n",

                                     (char *)op->value4, (char *)op->value5);

 			    xmlXPathPopFrame(ctxt, frame);

+			    ctxt->error = XPATH_UNDEF_PREFIX_ERROR;

                             return (total);

                         }

                         func = xmlXPathFunctionLookupNS(ctxt->context,

@@ -14016,6 +14021,7 @@

     }

     xmlGenericError(xmlGenericErrorContext,

                     "XPath: unknown precompiled operation %d\n", op->op);

+    ctxt->error = XPATH_INVALID_OPERAND;

     return (total);

 }