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]

(cherry picked from commit df5cba8)

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,
 					return -1;
 				}
 				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;
@@ -743,7 +744,8 @@ 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

@@ -852,3 +852,7 @@ lsp-ping-timestamp	lsp-ping-timestamp.pcap		lsp-ping-timestamp.out	-vv
 
 # lwres with "extra" bytes
 lwres_with_extra lwres_with_extra.pcap lwres_with_extra.out
+
+# 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