Fix access to null pointer in TiffParser

[piponazo]

Fix for the second issue reported in #590.

te->pValue() could be 0x0 with the toc file provided by the user.

I did not add a test to reproduce the issue because I do not have much spare time lately, and I think the fix is obvious enough.

[codecov]

Codecov Report

Merging #706 into 0.27 will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             0.27     #706     +/-   ##
=========================================
+ Coverage   62.44%   62.55%   +0.1%     
=========================================
  Files         156      156             
  Lines       21507    21506      -1     
=========================================
+ Hits        13431    13453     +22     
+ Misses       8076     8053     -23

Impacted Files	Coverage Δ
src/tiffimage_int.cpp	91.17% <100%> (+0.53%)	⬆️
src/tiffvisitor_int.cpp	87.31% <0%> (+1.36%)	⬆️
src/exiv2.cpp	69.45% <0%> (+1.37%)	⬆️

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[piponazo]

@tbeu if you are happy with the changes, could you approve? 😉

[D4N]

Thanks for fixing this!

[piponazo]

@D4N Thanks to you for the test! Next time, just push me a bit for doing the testing. Sometimes it's difficult to keep the motivation to do the things right 😉

[tbeu]

Ok now.