Issue #13355: Raise ValueError on random.triangular call with invalid params.

Initial patch by Yuriy Senko.

Lib/random.py

@@ -345,6 +345,16 @@ class Random(_random.Random):
         http://en.wikipedia.org/wiki/Triangular_distribution
 
         """
+        # Sanity check. According to the doc low must be less or equal to
+        # high. And mode should be somewhere between these bounds.
+        if low > high:
+            raise ValueError('high cannot be less then low.')
+        if mode is not None and (mode < low or mode > high):
+            raise ValueError('mode must be between low and high.')
+
+        if high == low:
+            return low
+
         u = self.random()
         c = 0.5 if mode is None else (mode - low) / (high - low)
         if u > c: