BUILD/MINOR: tools: fix build warning in debug_hexdump()

BUILD/MINOR: tools: fix build warning in debug_hexdump()

Commit 0ebb511 ("MINOR: tools: add a generic hexdump function for debugging")
introduced debug_hexdump() which is used to dump a memory area during
debugging sessions. This function can start at an unaligned offset and
uses a signed comparison to know where to start dumping from. But the
operation mixes signed and unsigned, making the test incorrect and causing
the following warnings to be emitted under Clang :

  src/standard.c:3775:14: warning: comparison of unsigned expression >= 0 is
        always true [-Wtautological-compare]
                          if (b + j >= 0 && b + j < len)
                              ~~~~~ ^  ~

Make "j" signed instead. At the moment this function is not used at all
so there's no impact. Thanks to Dmitry Sivachenko for reporting it. No
backport is needed.