target/common: Fix dasm traces truncation when using SnitchSim

[Raragyay]

When using the verification.py library and SnitchSim to run a simulation then read results from the kernel execution, the IPC thread will crash the whole program once SnitchSim has finished issuing the read commands. This is because the current check for EOF in ipc.cc erroneously checks for EOF before the fread attempt, instead of after the fread attempt. See the following StackOverflow for how this is usually corrected:

https://stackoverflow.com/questions/5431941/why-is-while-feoffile-always-wrong

Because of this erroneous check, the last operation (which is usually a read memory operation) will be doubly executed, causing the attempt to write to and flush rx to fail, since SnitchSim.py will have already closed the pipe and directory. As such, the program crashes and the .dasm traces are never fully written out.

This PR fixes the EOF check in ipc.cc to correctly check for EOF or errors after a read attempt instead of before.

[colluca]

Thanks a lot for your contribution @Raragyay!