Fix coverity detected issue in radv.c

Error: NULL_RETURNS (CWE-476): [#def114]
dnsmasq-2.86test7/src/radv.c:748: returned_null: "expand" returns "NULL" (checked 10 out of 11 times).
dnsmasq-2.86test7/src/radv.c:748: var_assigned: Assigning: "p" = "NULL" return value from "expand".
dnsmasq-2.86test7/src/radv.c:749: dereference: Dereferencing a pointer that might be "NULL" "p" when calling "memset". [Note: The source code implementation of the function has been overridden by a builtin model.]
dnsmasq-2.86test7/src/outpacket.c:83: example_checked: Example 1: "expand(len)" has its value checked in "p = expand(len)".
dnsmasq-2.86test7/src/outpacket.c:109: example_checked: Example 2: "expand(1UL)" has its value checked in "p = expand(1UL)".
dnsmasq-2.86test7/src/radv.c:269: example_checked: Example 3: "expand(16UL)" has its value checked in "ra = expand(16UL)".
dnsmasq-2.86test7/src/radv.c:363: example_checked: Example 4: "expand(32UL)" has its value checked in "opt = expand(32UL)".
dnsmasq-2.86test7/src/radv.c:708: example_checked: Example 5: "expand(32UL)" has its value checked in "opt = expand(32UL)".
 #  747|         int len = (maclen + 9) >> 3;
 #  748|         unsigned char *p = expand(len << 3);
 #  749|->       memset(p, 0, len << 3);
 #  750|         *p++ = ICMP6_OPT_SOURCE_MAC;
 #  751|         *p++ = len;

Error: NULL_RETURNS (CWE-476): [#def115]
dnsmasq-2.86test7/src/radv.c:748: returned_null: "expand" returns "NULL" (checked 10 out of 11 times).
dnsmasq-2.86test7/src/radv.c:748: var_assigned: Assigning: "p" = "NULL" return value from "expand".
dnsmasq-2.86test7/src/radv.c:750: dereference: Incrementing a pointer which might be null: "p".
dnsmasq-2.86test7/src/outpacket.c:83: example_checked: Example 1: "expand(len)" has its value checked in "p = expand(len)".
dnsmasq-2.86test7/src/outpacket.c:109: example_checked: Example 2: "expand(1UL)" has its value checked in "p = expand(1UL)".
dnsmasq-2.86test7/src/radv.c:269: example_checked: Example 3: "expand(16UL)" has its value checked in "ra = expand(16UL)".
dnsmasq-2.86test7/src/radv.c:363: example_checked: Example 4: "expand(32UL)" has its value checked in "opt = expand(32UL)".
dnsmasq-2.86test7/src/radv.c:708: example_checked: Example 5: "expand(32UL)" has its value checked in "opt = expand(32UL)".
 #  748|         unsigned char *p = expand(len << 3);
 #  749|         memset(p, 0, len << 3);
 #  750|->       *p++ = ICMP6_OPT_SOURCE_MAC;
 #  751|         *p++ = len;
 #  752|         memcpy(p, mac, maclen);

src/radv.c

@@ -746,6 +746,8 @@ static int add_lla(int index, unsigned int type, char *mac, size_t maclen, void
 	 add 7 to round up */
       int len = (maclen + 9) >> 3;
       unsigned char *p = expand(len << 3);
+      if (!p)
+	return 1;
       memset(p, 0, len << 3);
       *p++ = ICMP6_OPT_SOURCE_MAC;
       *p++ = len;