HTTP: Make Content-Range parsing more robust

This was exposed by OSS-Fuzz after the HTTP/0.9 changes in zeek/zeek#2851:
We do not check the result of parsing the from and last bytes of a
Content-Range header and would reference uninitialized values on the stack
if these were not valid.

This doesn't seem as bad as it sounds outside of yielding non-sensible values:
If the result was negative, we weird/bailed. If the result was positive, we
already had to treat it with suspicion anyway and the SetPlainDelivery()
logic accounts for that.

testing/btest/scripts/base/protocols/http/bad-content-range.zeek

@@ -0,0 +1,3 @@
+# @TEST-EXEC: zeek -b base/protocols/http -r $TRACES/http/http-bad-content-range-01.pcap
+# @TEST-EXEC: btest-diff http.log
+# @TEST-EXEC: btest-diff weird.log