mem,scons: Changed assert to panic_if in MessageBuffer
mem,scons: Changed assert to panic_if in MessageBuffer

The variable 'm_allow_zero_latency' was only used in an assert message in
`src/mem/ruby/network/MessageBuffer.cc`. This assert is stripped when
compiling to gem5.fast, resulting in the compilation failing with an
unused variable error.

This assert is better as a panic_if, which will not be stripped out
during the .fast compilation.

Change-Id: I5de74982fa42b3291899ddcf73f7140079e1ec3f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36697
Maintainer: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5