Update GdbSession.cs

fix CheckBreakpoint function.
bp.TraceExpression sometime is empty or null, if that's happen, gdb will report an error System.InvalidOperationException: -data-evaluate-expression: Usage: -data-evaluate-expression expression.
Merge pull request #432 from ekowahyudin/patch-1

Update GdbSession.cs
Merge branch 'master' of https://github.com/mono/monodevelop