Dies if command line found with no command

I have an email in the form of:

The first line of the body - starting with :, is improperly considered a Mailhandler command line. Due to lack of input checking, this would then attempt to set $node->'' to the string (since there's no command word here). The attached patch does a check to ensure that the command word exists before attempting to assign to $node. This patch DOES NOT attempt to add the erroneous line back to the body (primarily because I didn't investigate how). I'm not sure your feelings on this.