Fix warning when compiling with -O3:

/home/jmayer/work/wireshark/git/epan/dissectors/packet-ipp.c: In function ‘dissect_ipp’:
/home/jmayer/work/wireshark/git/epan/dissectors/packet-ipp.c:583:78: error: ‘name_val’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
             if ((name_length > 5) && name_val && !strcmp(name_val+name_length-5, "-time")) {
                                                                              ^
/home/jmayer/work/wireshark/git/epan/dissectors/packet-ipp.c:557:11: note: ‘name_val’ was declared here
     char *name_val;
           ^

Change-Id: I5e49596010572b285e4ca8f210cdcf49d56bfc10
Reviewed-on: https://code.wireshark.org/review/3572
Reviewed-by: Jörg Mayer <jmayer@loplof.de>