Avoid NullReferenceExeption in GetTextFromQuery (#346)

In rare cases, a thread-scheduling race condition can cause a thread's
state to be ThreadState.Stopped even though it hasn't yet executed its
`finally` block, which can result in result.StandardOutput or Error not
being assigned a string, leaving them null. Furthermore, the C# docs say
not to depend on ThreadState. So instead we check for the thread having
ended by seeing whether the `finally` block has assigned a value to its
Results property; until then, Results is null.

This should guarantee the safety of the existing GetTextFromQuery code,
but to be safe we also make sure it can turn null into an empty string.

src/LibChorus/Utilities/HgProcessOutputReader.cs

@@ -69,7 +69,7 @@ public bool Read(ref Process process, int secondsBeforeTimeOut, IProgress progre
 			}
 
 			//nb: at one point I (jh) tried adding !process.HasExited, but that made things less stable.
-			while ( _outputReader.ThreadState != ThreadState.Stopped && _errorReader.ThreadState != ThreadState.Stopped)
+			while (outputReaderArgs.Results == null || errorReaderArgs.Results == null)
 			{
 				DateTime end;
 				lock (this)

src/LibChorus/VcsDrivers/Mercurial/HgRepository.cs

@@ -468,13 +468,13 @@ private string GetTextFromQuery(string query, int secondsBeforeTimeoutOnLocalOpe
 		{
 			var result = ExecuteErrorsOk(query, secondsBeforeTimeoutOnLocalOperation);
 
-			var standardOutputText = result.StandardOutput.Trim();
+			var standardOutputText = result.StandardOutput?.Trim();
 			if (!string.IsNullOrEmpty(standardOutputText))
 			{
 				_progress.WriteVerbose(standardOutputText);
 			}
 
-			var standardErrorText = result.StandardError.Trim();
+			var standardErrorText = result.StandardError?.Trim();
 			if (!string.IsNullOrEmpty(standardErrorText))
 			{
 				_progress.WriteError(standardErrorText);
@@ -485,7 +485,7 @@ private string GetTextFromQuery(string query, int secondsBeforeTimeoutOnLocalOpe
 				_progress.WriteWarning("Hg Command {0} left lock", query);
 			}
 
-			return result.StandardOutput;
+			return result.StandardOutput ?? "";
 		}
 
 		/// <summary>