help flexelint understand what happens with memory

Flexelint sput warning 429 that the malloc() return value was neither
freed not returned.

This simplification makes it clear. The assertion on malloc() having
succeeded is in VSA_Build()

Ref #3275

lib/libvarnish/vsa.c

@@ -245,12 +245,7 @@ socklen_t sua_len(const struct sockaddr *sa)
 struct suckaddr *
 VSA_Malloc(const void *s, unsigned  sal)
 {
-	void *d;
-
-	d = malloc(vsa_suckaddr_len);
-	AN(d);
-
-	return (VSA_Build(d, s, sal));
+	return (VSA_Build(malloc(vsa_suckaddr_len), s, sal));
 }
 
 /* 'd' SHALL point to vsa_suckaddr_len aligned bytes of storage