Fix out-of-bounds heap access in internals.pyx. (#47935)

If blknos is empty, we unconditionally access blknos[start] where start
is 0. This is an out-of-bounds heap access that can be caught by
AddressSanitizer, but it's easy to avoid since
we are going to ignore the result anyway.

pandas/_libs/internals.pyx

@@ -469,12 +469,14 @@ def get_blkno_indexers(
 
     n = blknos.shape[0]
     result = list()
+
+    if n == 0:
+        return result
+
     start = 0
     cur_blkno = blknos[start]
 
-    if n == 0:
-        pass
-    elif group is False:
+    if group is False:
         for i in range(1, n):
             if blknos[i] != cur_blkno:
                 result.append((cur_blkno, slice(start, i)))