Fix `nan`-robust stream FPS (#6198)

Fix for Webcam stop working suddenly (Issue #6197)

utils/datasets.py

@@ -6,6 +6,7 @@ Dataloaders and dataset utils
 import glob
 import hashlib
 import json
+import math
 import os
 import random
 import shutil
@@ -308,8 +309,9 @@ class LoadStreams:
             assert cap.isOpened(), f'{st}Failed to open {s}'
             w = int(cap.get(cv2.CAP_PROP_FRAME_WIDTH))
             h = int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT))
-            self.fps[i] = max(cap.get(cv2.CAP_PROP_FPS) % 100, 0) or 30.0  # 30 FPS fallback
+            fps = cap.get(cv2.CAP_PROP_FPS)  # warning: may return 0 or nan
             self.frames[i] = max(int(cap.get(cv2.CAP_PROP_FRAME_COUNT)), 0) or float('inf')  # infinite stream fallback
+            self.fps[i] = max((fps if math.isfinite(fps) else 0) % 100, 0) or 30  # 30 FPS fallback
 
             _, self.imgs[i] = cap.read()  # guarantee first frame
             self.threads[i] = Thread(target=self.update, args=([i, cap, s]), daemon=True)