fix warning of level 4: (#971)
fix warning of level 4: (#971)

convert.h line130 : warning C4244 conversation from int to T possible loss of data
loot/yaml-cpp,jbeder/yaml-cpp,jbeder/yaml-cpp,dand-oss/yaml-cpp,loot/yaml-cpp,jbeder/yaml-cpp,graehl/yaml-cpp,jbeder/yaml-cpp,dand-oss/yaml-cpp,WrinklyNinja/yaml-cpp,dand-oss/yaml-cpp,dand-oss/yaml-cpp,loot/yaml-cpp,WrinklyNinja/yaml-cpp,graehl/yaml-cpp,graehl/yaml-cpp,graehl/yaml-cpp,WrinklyNinja/yaml-cpp,dand-oss/yaml-cpp,jbeder/yaml-cpp,WrinklyNinja/yaml-cpp,graehl/yaml-cpp,loot/yaml-cpp