Set pointers to NULL after free.
Set pointers to NULL after free.

Since Allplots.c reuses pointers with realloc, and the first step of
realloc is to free the old pointer if it isn't null this was leading to
a double free bug and segfault. By setting them to NULL immediately
after free we avoid this bug introduced in [2e9d25b2].

re #946 (0.5)

victor-lin/npact,NProfileAnalysisComputationalTool/npact,victor-lin/npact,NProfileAnalysisComputationalTool/npact,victor-lin/npact,NProfileAnalysisComputationalTool/npact,victor-lin/npact,victor-lin/npact,NProfileAnalysisComputationalTool/npact,NProfileAnalysisComputationalTool/npact