Fix build of dspdfviewer on 11.1

[adridg]

Summary

Build fails (while testing update to CMake, unrelated) with:

../hyperlinkarea.cpp:35:18: error: zero as null pointer constant [-Werror,-Wzero-as-null-pointer-constant]

if ( pixmap == 0 )
               ^
               nullptr

[tcberner]

Looks good to me.

[mat]

graphics/dspdfviewer/files/patch-hyperlinkarea.cpp

14–15 ↗

(On Diff #40163)

I think it should be == NULL. The problem it tells you is that you are comparing a pointer with an integer.