PerformanceProfiler: also instrument the class initializer

Static blocks and initializers of static field can take a huge amount
of time, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
PerformanceProfiler: handle invalid class definitions gracefully

Just as with missing dependencies, invalid dependencies can be ignored:
they were never executed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>