AP_Filesystem: fixed stat call for @SYS files

libraries/AP_Filesystem/AP_Filesystem_Sys.cpp

@@ -226,7 +226,11 @@ int AP_Filesystem_Sys::stat(const char *pathname, struct stat *stbuf)
         stbuf->st_size = 0; // just a placeholder value
         return 0;
     }
-    int8_t pos = file_in_sysfs(&pathname[1]);
+    const char *pathname_noslash = pathname;
+    if (pathname[0] == '/') {
+        pathname_noslash = &pathname[1];
+    }
+    int8_t pos = file_in_sysfs(pathname_noslash);
     if (pos < 0) {
         errno = ENOENT;
         return -1;