#595 use allocate memory before null check

[wjq]

In the latest version of GraphicsMagick, allocated memory is used without null pointer check which can cause a null pointer de-reference bug.
In codes/pdb.c line 673,674,675;
In codes/render.c line 2506 and following lines;
In magick/segment.c line 399 and following lines;
In magick/xwindow.c line 4669 and following lines;

[Bob Friesenhahn]

• I have implemented a fix for pdb.c.
• The source code in magick/render.c line 2506 is specifically to deal with memory allocation failure so I wonder what you mean by "latest version of GraphicsMagick".
• I have impemented a fix for segment.c
• I have implemented a fix for xwindow.c

The fixes are implemented in changeset 15944:4188ef30df01. Thanks for inspecting the code and reporting these issues.