correct the pos's size and then fix the following two compile warnings:

/Users/lgq/github/sysdig/userspace/libsinsp/filterchecks.cpp:209:11: warning: comparison of constant 18446744073709551615
      with expression of type 'uint32_t' (aka 'unsigned int') is always true [-Wtautological-constant-out-of-range-compare]
                        if(pos != string::npos)
                           ~~~ ^  ~~~~~~~~~~~~
/Users/lgq/github/sysdig/userspace/libsinsp/filterchecks.cpp:315:11: warning: comparison of constant 18446744073709551615
      with expression of type 'uint32_t' (aka 'unsigned int') is always true [-Wtautological-constant-out-of-range-compare]
                        if(pos != string::npos)
                           ~~~ ^  ~~~~~~~~~~~~
Merge pull request #104 from guanqun/fix-rfind-warnings

correct the pos's size and then fix the following two compile warnings: