Reject + sign when checking -ipcfd

src/ipc/process.cpp

@@ -55,9 +55,11 @@ public:
         // in combination with other arguments because the parent process
         // should be able to control the child process through the IPC protocol
         // without passing information out of band.
-        if (!ParseInt32(argv[2], &fd)) {
+        const auto maybe_fd{ToIntegral<int32_t>(argv[2])};
+        if (!maybe_fd) {
             throw std::runtime_error(strprintf("Invalid -ipcfd number '%s'", argv[2]));
         }
+        fd = *maybe_fd;
         return true;
     }
     int connect(const fs::path& data_dir,