Fix negative tail

Author: savalet

include/dataframe.h

@@ -16,6 +16,7 @@
 
     #define WC(fd, str) write(fd, str, sizeof str - 1)
     #define IDX_OF(array, i, mem_s) (array + ((i) * (mem_s)))
+    #define ABS(v) (((v) < (0)) ? -(v) : (v))
 
 typedef enum {
     BOOL,

src/df_tail.c

@@ -16,8 +16,7 @@ dataframe_t *df_tail(dataframe_t *df, int nb_rows)
     dataframe_t *new_df;
     void ***copy_src_data = df->data;
 
-    if (!nb_rows)
-        return NULL;
+    nb_rows = ABS(nb_rows);
     new_df = malloc(sizeof(dataframe_t));
     if (new_df == NULL)
         return NULL;