[xml] Omitting prefix in XPATH (libxml2) leads to segmentation fault

[Shivan Kaul Sahib (ssahib)]

I installed C libxml2 version 2.9.4 library on my Mac (Darwin <hostname> 16.7.0 
Darwin Kernel Version 16.7.0: Thu Jun 15 17:36:27 PDT 2017; 
root:xnu-3789.70.16~2/RELEASE_X86_64 x86_64) using the standard `./configure ; 
make ; make install` trifecta of commands. On trying to run the `testXPath` 
command with the `-tree` option on an XPATH without a prefix, I get a 
segmentation fault.

    ~/home/libxml2-2.7.2 $ ./testXPath -tree "/abcde"
    In testXPath function...
    Compiled Expression : 0 elements
      END
        END
          END
            END
              END
                END
                  END
                    END
                      END
                        END
                          END
                            END
                              END
                                END
                                  END
                                    END
                                      END
                                        END
                                          END
                                            END
                                              END
                                                END
                                                  END
                                                    END
                                                      END
                                                      END
                                                      END
                                                      END
                                                      END
                                                      END
                                                      END
    ...etc until:
    Segmentation fault: 11


I dug in a bit into the `testXPath.c` code, and the compilation of the XPATH 
seems to be faulty.

    comp = xmlXPathCompile(BAD_CAST str);

The `xmlXPathCompExprPtr` returned has its `last` member set to be `-1` if I 
don't supply a prefix. This causes the call to `xmlXPathDebugDumpCompExpr()` to 
glitch out.

Having a prefix in the xpath doesn't cause the fault. Am I doing something 
wrong?

Thanks,
Shivan

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