ms_nrtp: bound NRBF string lengths to 31 bits

[KernelClint]

NRTP reassembly decodes an application/octet-stream body as .NET Remoting Binary Format. NRBF
encodes a string's length as a variable-length integer, seven bits per byte, and the format caps it
at five bytes and 31 bits.

scapy/layers/ms_nrtp.py:333-350
uses the generic extended-integer decoder for that field, and the generic decoder has no cap. It
keeps consuming continuation bytes and shifting a Python integer that grows with the input, so the
work is quadratic in the number of bytes supplied — all before it looks for the string data, which
does not exist.

The change enforces the limit the format already defines, in the NRBF-specific field, before the
generic decoder runs:

+    def getfield(self, pkt, s):
+        if len(s) >= 5 and min(s[:4]) >= 0x80 and s[4] > 0x07:
+            raise Scapy_Exception("NRBF string length exceeds 31 bits")
+        return super(MSBExtendedFieldLen, self).getfield(pkt, s)

The largest valid encoding, ff ff ff ff 07, is still accepted; only encodings the format does not
permit are rejected.

The added regression parses that maximum encoding and then an over-wide one, asserting the first is
accepted and the second raises. Without the source change the NRTP suite reports 2 passed and 1
failed; with it, 3 passed and 0 failed.

Performance was measured on one computer, before and after the fix: parsing a valid NRBF body took
4,430.4 ns before and 4,598.5 ns after. Repeat runs of this test moved by about 9%, which is wide
enough that only a large change would show — this is not one.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.64%. Comparing base (b3bbcc8) to head (d4b745d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5135   +/-   ##
=======================================
  Coverage   80.63%   80.64%           
=======================================
  Files         390      390           
  Lines       96936    96941    +5     
=======================================
+ Hits        78168    78181   +13     
+ Misses      18768    18760    -8     

Files with missing lines	Coverage Δ
scapy/layers/ms_nrtp.py	82.08% <100.00%> (+0.29%)	⬆️

... and 10 files with indirect coverage changes

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