Off by one in writing the last entry when parsing input

src/matrix.c

@@ -143,7 +143,7 @@ matrix_t ParseMatrix(size_t row_len) {
   // Write the last entry if necessary
   if (!spaces) {
     data_t value = as_matrix_entry(scan_str, scan_len);
-    head[len] = value;
+    head[len - 1] = value;
   }
 
   // Input sanitization: confirm that the number of entries read is a multiple