Yet another fix for the M999 resend issue

Should hopefully now be also fixed in case of a newly established connection with the printer, which was a regression due to the fix of the resending code.

octoprint/util/comm.py

@@ -578,6 +578,7 @@ class MachineCom(object):
 		sdStatusRequestTimeout = timeout
 		startSeen = not settings().getBoolean(["feature", "waitForStartOnConnect"])
 		while True:
+			try:
 				line = self._readline()
 				if line == None:
 					break
@@ -766,6 +767,13 @@ class MachineCom(object):
 								self._sendNext()
 						elif "resend" in line.lower() or "rs" in line:
 							self._handleResendRequest(line)
+			except:
+				self._logger.exception("Something crashed inside the serial connection loop, please report this in OctoPrint's bug tracker:")
+
+				errorMsg = "See octoprint.log for details"
+				self._log(errorMsg)
+				self._errorValue = errorMsg
+				self._changeState(self.STATE_ERROR)
 		self._log("Connection closed, closing down monitor")
 
 	def _handleResendRequest(self, line):
@@ -778,7 +786,7 @@ class MachineCom(object):
 
 		if lineToResend is not None:
 			self._resendDelta = self._currentLine - lineToResend
-			if self._resendDelta > len(self._lastLines):
+			if self._resendDelta > len(self._lastLines) or len(self._lastLines) == 0:
 				self._errorValue = "Printer requested line %d but no sufficient history is available, can't resend" % lineToResend
 				self._logger.warn(self._errorValue)
 				if self.isPrinting():