Fix substring indexing crash

* With the right input the do/while loop is executed with size == 0 leading to
  an unsigned integer underflow

indexer-worker: Error: terminate called after throwing an instance of 'std::length_error'
indexer-worker: Error:   what():  basic_string::_M_create
indexer-worker(user)<640260><mmnkFF9EdWYAxQkAaoL48g:6A7sFV9EdWYExQkAaoL48g>: Fatal: master: service(indexer-worker): child 640260 killed with signal 6 (core dumped)

src/fts-backend-flatcurve-xapian.cpp

@@ -1284,7 +1284,7 @@ fts_flatcurve_xapian_index_header(struct flatcurve_fts_backend_update_context *c
 		size -= csize;
 	} while (fuser->set.substring_search &&
 		(size >= 0) &&
-		(uni_utf8_strlen(p) >= fuser->set.min_term_size));
+		(uni_utf8_strlen_n(p, size) >= fuser->set.min_term_size));
 }
 
 void
@@ -1315,7 +1315,7 @@ fts_flatcurve_xapian_index_body(struct flatcurve_fts_backend_update_context *ctx
 		size -= csize;
 	} while (fuser->set.substring_search &&
 		(size >= 0) &&
-		(uni_utf8_strlen(p) >= fuser->set.min_term_size));
+		(uni_utf8_strlen_n(p, size) >= fuser->set.min_term_size));
 }
 
 void fts_flatcurve_xapian_delete_index(struct flatcurve_fts_backend *backend)