Avoid uninitialized memory access

As reported by https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=68258

src/mat5.c

@@ -3245,6 +3245,9 @@ Mat_VarRead5(mat_t *mat, matvar_t *matvar)
                 size_t nbytes = 0;
                 err = Mul(&nbytes, nelems, matvar->data_size);
                 if ( err || nbytes > matvar->nbytes ) {
+                    free(matvar->data);
+                    matvar->data = NULL;
+                    matvar->nbytes = 0;
                     break;
                 }
             }