Fix bug #338666: "Kalzium crashes when selecting >File >Export Data..."

Problem is easy to spot with valgrind:
The private ExportDialog *m_exportDialog is not explicitely initialized
with 0, so later calling "if (!m_exportDialog)" may fail.

This accidentally slipped in with a header cleanup (commit d36687d5...)

Easy fix: initialize the pointer with 0 just like all the others.

BUG: 338666
FIXED-IN: 4.14.2