Add bounds checking in find_step

Check the value of cur_mini_step is sane before using it as an index to
mini_step array.

Signed-off-by: Andrew Price <anprice@redhat.com>

plot.c

diff --git a/plot.c b/plot.c
index 971a253..d486f29 100644
--- a/plot.c
+++ b/plot.c
@@ -530,10 +530,12 @@
 	/* Round to power of 10 */
 	step = exp(floor(log(step) / log10) * log10);
 	/* Scale down step to provide enough ticks */
-	while ((last - first) / (step * mini_step[cur_mini_step]) > num_ticks
-	       && cur_mini_step < TICK_MINI_STEPS)
+	while (cur_mini_step < TICK_MINI_STEPS
+	       && (last - first) / (step * mini_step[cur_mini_step]) > num_ticks)
 		cur_mini_step++;
-	step *= mini_step[cur_mini_step - 1];
+
+	if (cur_mini_step > 0)
+		step *= mini_step[cur_mini_step - 1];
 
 	return step;
 }