BUILD/MINOR: stats: fix build warning due to condition always true

BUILD/MINOR: stats: fix build warning due to condition always true

Dmitry Sivachenko reported the following harmless build warning using Clang :

  src/dumpstats.c:5196:48: warning: address of array 'strm_li(sess)->proto->name'
        will always evaluate to 'true' [-Wpointer-bool-conversion]
    ...strm_li(sess) && strm_li(sess)->proto->name ? strm_li(sess)->proto->nam...
                     ~~ ~~~~~~~~~~~~~~~~~~~~~~^~~~
proto->name cannot be null here as it's the protocol name which is stored
directly in the structure.

The same case is present in 1.5 though the code changed.