Fix potential crash in BreakpointModel::removeRows

This crashed for me when row == 0 && count == 0.
Now validates input.

Fixes the crash on the CI:
QFATAL : GDBDebugger::GdbTest::testStdOut() ASSERT: "last >= first" in
file kernel/qabstractitemmodel.cpp, line 2467
gdbtest: Fix one of the test methods

Make Breakpoint and BreakpointModel more robust.

Output:
QFATAL :
GDBDebugger::GdbTest::testInsertAndRemoveBreakpointWhileRunning()
ASSERT: "idx.isValid()" in file
/home/krf/devel/src/kdevplatform/debugger/breakpoint/breakpointmodel.cpp,
line 338