snmp: keep printing results after an ICMP answer

[KernelClint]

snmpget() can query several targets in one call and prints what each returns. Some answers are
not SNMP at all: Scapy matches an ICMP error to the probe that provoked it, so an unreachable host
or router appears in the answered set.

The result loop at
scapy/layers/snmp.py:290-317
prints that error and then returns from the function rather than moving to the next answer.
Everything before the ICMP answer in the iteration has already printed; everything after it is
discarded. The results were collected — they are simply never shown.

The change continues instead of returning:

         if ICMP in r.answer:
             print("%s: %s" % (r.answer.src, r.answer.sprintf("%ICMP.type%")))
-            return
+            continue

The diagnostic still prints, so nothing about the error case is lost.

The added regression builds a multi-target result set with an ICMP answer ahead of a valid SNMP
answer and asserts both appear in the output. Restoring the return makes it fail.

Performance was measured on one computer, before and after the fix: a representative call took
113.5 µs before and 114.3 µs after. Repeat runs moved by about 2%, so that difference is smaller
than the test can distinguish.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.64%. Comparing base (b3bbcc8) to head (e5ab25f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5128      +/-   ##
==========================================
+ Coverage   80.63%   80.64%   +0.01%     
==========================================
  Files         390      390              
  Lines       96936    96936              
==========================================
+ Hits        78168    78178      +10     
+ Misses      18768    18758      -10     

Files with missing lines	Coverage Δ
scapy/layers/snmp.py	89.06% <100.00%> (+4.68%)	⬆️

... and 3 files with indirect coverage changes

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