Backport PR #55726 on branch 2.1.x (fix segfault with tz_localize_to_utc) (#55796)

Backport PR #55726: fix segfault with tz_localize_to_utc

Co-authored-by: William Ayd <[email protected]>

Author: meeseeksmachine

pandas/_libs/tslibs/tzconversion.pyx

@@ -332,13 +332,16 @@ timedelta-like}
     # Shift the delta_idx by if the UTC offset of
     # the target tz is greater than 0 and we're moving forward
     # or vice versa
-    first_delta = info.deltas[0]
-    if (shift_forward or shift_delta > 0) and first_delta > 0:
-        delta_idx_offset = 1
-    elif (shift_backward or shift_delta < 0) and first_delta < 0:
-        delta_idx_offset = 1
-    else:
-        delta_idx_offset = 0
+    # TODO: delta_idx_offset and info.deltas are needed for zoneinfo timezones,
+    # but are not applicable for all timezones. Setting the former to 0 and
+    # length checking the latter avoids UB, but this could use a larger refactor
+    delta_idx_offset = 0
+    if len(info.deltas):
+        first_delta = info.deltas[0]
+        if (shift_forward or shift_delta > 0) and first_delta > 0:
+            delta_idx_offset = 1
+        elif (shift_backward or shift_delta < 0) and first_delta < 0:
+            delta_idx_offset = 1
 
     for i in range(n):
         val = vals[i]