Fix an always-true check

Fixed asprintf return value check, in order to properly catch
error conditions. This has been catch by clang -Wtautological-compare:

pw-pbkdf2.c:132:17: warning: comparison of unsigned expression < 0 is always false
        if(msg->bv_len < 0){
           ~~~~~~~~~~~ ^ ~

Signed-off-by: Luca Bruno <luca.bruno@rocket-internet.de>