Fix divide by zero in sinefit

nilmtools/sinefit.py

@@ -96,8 +96,11 @@ def process(data, interval, args, insert_function, final):
     rows = data.shape[0]
 
     # Estimate sampling frequency from timestamps
-    fs = (rows-1) / (timestamp_to_seconds(data[-1][0]) -
-                     timestamp_to_seconds(data[0][0]))
+    ts_min = timestamp_to_seconds(data[0][0])
+    ts_max = timestamp_to_seconds(data[-1][0])
+    if ts_min >= ts_max:
+        return 0
+    fs = (rows-1) / (ts_max - ts_min)
 
     # Pull out about 3.5 periods of data at once;
     # we'll expect to match 3 zero crossings in each window