Remove unnecessary assert in ICMP analyzer (addresses #822).

The ICMP/ICMPv6 analyzers function correctly when full packets have
not been captured, but everything up to and including the ICMP header
is there (e.g. the functions that inspect ICMP error message context
correctly check the caplen to see if more info can be extracted).

The "Should have been caught earlier already." comment may have referred
to NetSessions::CheckHeaderTrunc, which works as intended to catch cases
where the ICMP header is not there in full, but then the assert was
still not correctly formulated for that...

Also changed the ICMP checksum calculation to not occur when the full
packet has not been captured, which seems consistent with what the UDP
analysis does.

testing/btest/Baseline/core.truncation/output

@@ -22,3 +22,11 @@
 #fields	ts	uid	id.orig_h	id.orig_p	id.resp_h	id.resp_p	name	addl	notice	peer
 #types	time	string	addr	port	addr	port	string	string	bool	string
 1334094648.590126	-	-	-	-	-	truncated_IP	-	F	bro
+#separator \x09
+#set_separator	,
+#empty_field	(empty)
+#unset_field	-
+#path	weird
+#fields	ts	uid	id.orig_h	id.orig_p	id.resp_h	id.resp_p	name	addl	notice	peer
+#types	time	string	addr	port	addr	port	string	string	bool	string
+1338328954.078361	-	-	-	-	-	internally_truncated_header	-	F	bro

testing/btest/core/truncation.test

@@ -6,4 +6,17 @@
 # @TEST-EXEC: cat weird.log >> output
 # @TEST-EXEC: bro -r $TRACES/trunc/ip6-ext-trunc.pcap
 # @TEST-EXEC: cat weird.log >> output
+
+# If an ICMP packet's payload is truncated due to too small snaplen,
+# the checksum calculation is bypassed (and Bro doesn't crash, of course).
+
+# @TEST-EXEC: rm -f weird.log
+# @TEST-EXEC: bro -r $TRACES/trunc/icmp-payload-trunc.pcap
+# @TEST-EXEC: test ! -e weird.log
+
+# If an ICMP packet has the ICMP header truncated due to too small snaplen,
+# an internally_truncated_header weird gets generated.
+
+# @TEST-EXEC: bro -r $TRACES/trunc/icmp-header-trunc.pcap
+# @TEST-EXEC: cat weird.log >> output
 # @TEST-EXEC: btest-diff output