client: Silence a gcc-7.1,0 warning.

client: Silence a gcc-7.1,0 warning.

This version of gcc complains because a case statement in
client_pre_select() may fall through. The fallthrough is intentional,
however, and this is annotated with a comment. gcc-7 scans the comments
in a switch statement and suppresses the warning if it recognizes
a comment which says that fallthrough is intended. But for this to
happen, we must spell it correctly, and without whitespace.

After this commit, gcc-7 compiles the tree with no warnings.