scan-build: avoid undef behaior in tor_inet_pton

If we had an address of the form "1.2.3.4" and we tried to pass it to
tor_inet_pton with AF_INET6, it was possible for our 'eow' pointer to
briefly move backwards to the point before the start of the string,
before we moved it right back to the start of the string.  C doesn't
allow that, and though we haven't yet hit a compiler that decided to
nuke us in response, it's best to fix.

So, be more explicit about requiring there to be a : before any IPv4
address part of the IPv6 address.  We would have rejected addresses
without a : for not being IPv6 later on anyway.

src/common/compat.c

@@ -2195,8 +2195,10 @@ tor_inet_pton(int af, const char *src, void *dst)
     else {
       unsigned byte1,byte2,byte3,byte4;
       char more;
-      for (eow = dot-1; eow >= src && TOR_ISDIGIT(*eow); --eow)
+      for (eow = dot-1; eow > src && TOR_ISDIGIT(*eow); --eow)
         ;
+      if (*eow != ':')
+        return 0;
       ++eow;
 
       /* We use "scanf" because some platform inet_aton()s are too lax

src/test/test_addr.c

@@ -346,6 +346,9 @@ test_addr_ip6_helpers(void)
   test_pton6_bad("a:::b:c");
   test_pton6_bad(":::a:b:c");
   test_pton6_bad("a:b:c:::");
+  test_pton6_bad("1.2.3.4");
+  test_pton6_bad(":1.2.3.4");
+  test_pton6_bad(".2.3.4");
 
   /* test internal checking */
   test_external_ip("fbff:ffff::2:7", 0);