There have been cases observed where 'p QArrayData::shared_null' finds
valid symbols that are not found using gdb.lookup_symbols. The cause
for that is unknown.

Apply an expensive workaround by checking for (the equivalent of)
a working 'p QArrayData::shared_null' but execute it only when
a libQt5Core was found. This keeps the overhead for non-Qt setups
at a bearable (unsuccessful) iteration over known shared object
names.

Change-Id: Id398673b938d3c3a72c24317abdbefbe793e54df
Reviewed-by: Christian Stenger <christian.stenger@qt.io>