SNMP: Fix two undefined behaviors

When converting an integer from ASN.1, use an unsigned value
for the partial result and assign it to the integer part of
the union at the end, to avoid shifting a negative number left.

print-snmp.c:545:19: runtime error: left shift of negative value -1
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior print-snmp.c:545:19

OID elements are unsigned; a large-enough oid value could result
in the undefined behavior of shifting a signed integer left through
the sign bit, so simply store them as unsigned.

print-snmp.c:751:11: runtime error: left shift of 268435455 by 7 places
  cannot be represented in type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior print-snmp.c:751:11

[Part of the PR the-tcpdump-group#1012]

CHANGES

@@ -27,6 +27,7 @@ DayOfTheWeek, Month DD, YYYY / The Tcpdump Group
       BGP: Add support for BGP Role capability and OTC attribute
       Arista: Use the test .pcap file from pull request #955 (HwInfo).
       NFLOG: Use correct AF code points on all OSes.
+      SNMP: Fix two undefined behaviors.
     User interface:
       Add optional unit suffix on -C file size.
       Add --print-sampling to print every Nth packet instead of all.

print-snmp.c

@@ -66,6 +66,7 @@
 
 #include <stdio.h>
 #include <string.h>
+#include <limits.h>
 
 #ifdef USE_LIBSMI
 #include <smi.h>
@@ -531,7 +532,7 @@ asn1_parse(netdissect_options *ndo,
 				break;
 
 			case INTEGER: {
-				int32_t data;
+				uint32_t data;
 				elem->type = BE_INT;
 				data = 0;
 
@@ -540,7 +541,7 @@ asn1_parse(netdissect_options *ndo,
 					goto invalid;
 				}
 				if (GET_U_1(p) & ASN_BIT8)	/* negative */
-					data = -1;
+					data = UINT_MAX;
 				for (i = elem->asnlen; i != 0; p++, i--)
 					data = (data << ASN_SHIFT8) | GET_U_1(p);
 				elem->data.integer = data;
@@ -726,7 +727,8 @@ asn1_print(netdissect_options *ndo,
 		break;
 
 	case BE_OID: {
-		int o = 0, first = -1;
+		int first = -1;
+		uint32_t o = 0;
 
 		p = (const u_char *)elem->data.raw;
 		i = asnlen;

tests/TESTLIST

@@ -918,3 +918,7 @@ NHRP_registration		NHRP_registration.pcap		NHRP_registration.out	-v
 NHRP-responder-address		NHRP-responder-address.pcap	NHRP-responder-address.out	-v
 nhrp-trace			nhrp-trace.pcap			nhrp-trace.out	-v
 nhrp				nhrp.pcap			nhrp.out	-v
+
+# Undefined behavior tests
+ip-snmp-leftshift-unsigned ip-snmp-leftshift-unsigned.pcap ip-snmp-leftshift-unsigned.out
+ip6-snmp-oid-unsigned ip6-snmp-oid-unsigned.pcap ip6-snmp-oid-unsigned.out

tests/ip-snmp-leftshift-unsigned.out

@@ -0,0 +1 @@
+    1  14:35:45.695106 IP truncated-ip - 34734 bytes missing! 10.0.0.0.162 > 154.1.214.234.65535:  [!init SEQ]-1

tests/ip6-snmp-oid-unsigned.out

@@ -0,0 +1 @@
+    1  12:36:48.416500 IP6 fe80::20c:29ff:fe9b:a15d.161 > fe80::20c:0:0:0.546:  [!init SEQ].1.11.1.99.0.0.0.0.0.0.4.4.71.8327.1936855.0.1.0.14.0.1.0.1.24.14347.63.0.12.41.57.14824.0.2.0.14.0.1.0.1.24.1821339.0.12.41.446675.0.56.0.61.0.11.0.16.42.1.0.0.4294967168.0.0.0.0.0.0.0.0.0.0.1.0.2.3.110.116.112.7.101.120.97