Fix for bug #31438: updatexml still crashes

Problem: check missed if the second updatexml() argument is valid.

Fix: check it.

mysql-test/r/xml.result

@@ -1012,3 +1012,14 @@ select ExtractValue('<a>a</a>', '/a[@x=@y0123456789_0123456789_0123456789_012345
 ERROR HY000: XPATH error: comparison of two nodesets is not supported: '=@y0123456789_0123456789_0123456'
 select ExtractValue('<a>a</a>', '/a[@x=$y0123456789_0123456789_0123456789_0123456789]');
 ERROR HY000: Unknown XPATH variable at: '$y0123456789_0123456789_01234567'
+select updatexml(NULL, 1, 1), updatexml(1, NULL, 1), updatexml(1, 1, NULL);
+updatexml(NULL, 1, 1)	updatexml(1, NULL, 1)	updatexml(1, 1, NULL)
+NULL	NULL	NULL
+select updatexml(NULL, NULL, 1), updatexml(1, NULL, NULL), 
+updatexml(NULL, 1, NULL);
+updatexml(NULL, NULL, 1)	updatexml(1, NULL, NULL)	updatexml(NULL, 1, NULL)
+NULL	NULL	NULL
+select updatexml(NULL, NULL, NULL);
+updatexml(NULL, NULL, NULL)
+NULL
+End of 5.1 tests

mysql-test/t/xml.test

@@ -533,3 +533,14 @@ select UpdateXML('<a>a</a>',repeat('a b ',1000),'');
 select ExtractValue('<a>a</a>', '/a[@x=@y0123456789_0123456789_0123456789_0123456789]');
 --error 1105
 select ExtractValue('<a>a</a>', '/a[@x=$y0123456789_0123456789_0123456789_0123456789]');
+
+#
+# Bug #31438: updatexml still crashes
+#
+
+select updatexml(NULL, 1, 1), updatexml(1, NULL, 1), updatexml(1, 1, NULL);
+select updatexml(NULL, NULL, 1), updatexml(1, NULL, NULL), 
+       updatexml(NULL, 1, NULL);
+select updatexml(NULL, NULL, NULL);
+
+--echo End of 5.1 tests

sql/item_xmlfunc.cc

@@ -2580,7 +2580,8 @@ void Item_xml_str_func::fix_length_and_dec()
     return;
   }
 
-  xp= args[1]->val_str(&tmp);
+  if (!(xp= args[1]->val_str(&tmp)))
+    return;
   my_xpath_init(&xpath);
   xpath.cs= collation.collation;
   xpath.debug= 0;