Properly check for end of data when looking for the tracking id

Co-authored-by: Brice Figureau <brice-puppet@daysofwonder.com>

ngx_http_uploadprogress_module.c

                 i = 1;
                 break;
             }
-            else if (!len) {
-                break;
-            }
-        } while (p++);
+            p++;
+        } while (len > 0);
 
         if (i) {
             start_p = p += upcf->header.len + 1;
                 i = 1;
                 break;
             }
-            else if (!len) {
-                break;
-            }
-        } while (p++);
+            p++;
+        } while (len > 0);
 
         if (i) {
             start_p = p += upcf->header_mul.len + 1;