Previously this function relied on having ATTRIBUTE_NONNULL(1) in its
prototype rather than explicitly checking for a null
ifname. Unfortunately, ATTRIBUTE_NONNULL is just a hint to the
optimizer and code analyzers like Coverity, it doesn't actually check
anything at execution time, so the result was possible warnings from
Coverity, along with the possibility of null dereferences when ifname
wasn't available.

This patch removes the ATTRIBUTE_NONNULL from the prototype, and
checks ifname inside the function, logging an error if it's NULL (once
we've determined that the user really is trying to set a bandwidth).