pySim/ts_51_011: Properly re-compute ScAddr length

EF.SMSP contains up to two addresses: Both are stored in a fixed-length
field of 12 octets.  However, the actually used size depends on the
number of digits in the respective number.  Let's compute that length
field properly

Change-Id: Idef54a3545d1a5367a1efa2f0a6f7f0c1f860105

pySim/utils.py

@@ -526,6 +526,13 @@ def expand_hex(hexstring, length):
     # no change
     return hexstring
 
+def bytes_for_nibbles(num_nibbles: int) -> int:
+    """compute the number of bytes needed to store the given number of nibbles."""
+    n_bytes = num_nibbles // 2
+    if num_nibbles & 1:
+        n_bytes += 1
+    return n_bytes
+
 
 def boxed_heading_str(heading, width=80):
     """Generate a string that contains a boxed heading."""