Fix warnings: 'may be used uninitialized in this function'

icetime.cc: In member function ‘double TimingAnalysis::report(std::__cxx11::string)’:
icetime.cc:1095:15: warning: ‘last_time’ may be used uninitialized in this function [-Wmaybe-uninitialized]
        fprintf(frpt, "%10.3f ns ..%7.3f ns %s\n", first_time, last_time, last_net.c_str());
        ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
icetime.cc:1095:15: warning: ‘first_time’ may be used uninitialized in this function [-Wmaybe-uninitialized]
1 file changed