It looks to me like mrconvert crashed out, and hence didn’t provide any input for dwiextract to process, which would explain the messages that we do see (the shell typically won’t report a segfault if the command is not last in the pipeline). The most likely reason for mrconvert to crash is an unhandled memory allocation failure, due to running out of RAM, as has been reported a few times recently (here, here, and here).
Just to check this: how much RAM does your system have, and how big are the images you’re trying to process…?