Fix bvar's bad return value (#3151)

Author: songqing

src/bvar/default_variables.cpp

@@ -107,7 +107,7 @@ static bool read_proc_status(ProcStat &stat) {
             ",tpgid,flags,pri,nice | tail -n1", (long)pid);
     if (butil::read_command_output(oss, cmdbuf) != 0) {
         LOG(ERROR) << "Fail to read stat";
-        return -1;
+        return false;
     }
     const std::string& result = oss.str();
     if (sscanf(result.c_str(), "%d %d %d %d"
@@ -230,7 +230,7 @@ static bool read_proc_memory(ProcMemory &m) {
     snprintf(cmdbuf, sizeof(cmdbuf), "ps -p %ld -o rss=,vsz=", (long)pid);
     if (butil::read_command_output(oss, cmdbuf) != 0) {
         LOG(ERROR) << "Fail to read memory state";
-        return -1;
+        return false;
     }
     const std::string& result = oss.str();
     if (sscanf(result.c_str(), "%ld %ld", &m.resident, &m.size) != 2) {
@@ -292,7 +292,7 @@ static bool read_load_average(LoadAverage &m) {
     std::ostringstream oss;
     if (butil::read_command_output(oss, "sysctl -n vm.loadavg") != 0) {
         LOG(ERROR) << "Fail to read loadavg";
-        return -1;
+        return false;
     }
     const std::string& result = oss.str();
     if (sscanf(result.c_str(), "{ %lf %lf %lf }",