Add memcpy_safe

ISO/IEC 9899:1999 §7.21.1/2 says:
Where an argument declared as size_t n specifies the length of the array
for a function, n can have the value zero on a call to that
function. Unless explicitly stated otherwise in the description of a
particular function in this subclause, pointer arguments on such a call
shall still have valid values, as described in 7.1.4.

In base64_append_width memcpy was called as memcpy(x, NULL, 0).  GCC 4.9
started making use of this and assumes This worked fine under -O0, but
does something strange under -O3.

This patch fixes a bug in base64_append_width(), fixes a possible bug in
journal_file_append_entry_internal(), and makes use of the new function
to simplify the code in other places.

src/libsystemd/sd-bus/bus-control.c

@@ -1133,8 +1133,7 @@ static int add_name_change_match(sd_bus *bus,
                 item->name_change.old_id.id = old_owner_id;
                 item->name_change.new_id.id = new_owner_id;
 
-                if (name)
-                        memcpy(item->name_change.name, name, l);
+                memcpy_safe(item->name_change.name, name, l);
 
                 /* If the old name is unset or empty, then
                  * this can match against added names */

src/libsystemd/sd-bus/bus-message.c

@@ -2633,8 +2633,7 @@ _public_ int sd_bus_message_append_array(
         if (r < 0)
                 return r;
 
-        if (size > 0)
-                memcpy(p, ptr, size);
+        memcpy_safe(p, ptr, size);
 
         return 0;
 }

src/libsystemd/sd-bus/bus-socket.c

@@ -352,7 +352,7 @@ static int bus_socket_auth_write(sd_bus *b, const char *t) {
         if (!p)
                 return -ENOMEM;
 
-        memcpy(p, b->auth_iovec[0].iov_base, b->auth_iovec[0].iov_len);
+        memcpy_safe(p, b->auth_iovec[0].iov_base, b->auth_iovec[0].iov_len);
         memcpy(p + b->auth_iovec[0].iov_len, t, l);
 
         b->auth_iovec[0].iov_base = p;
@@ -789,7 +789,7 @@ int bus_socket_write_message(sd_bus *bus, sd_bus_message *m, size_t *idx) {
 
         n = m->n_iovec * sizeof(struct iovec);
         iov = alloca(n);
-        memcpy(iov, m->iovec, n);
+        memcpy_safe(iov, m->iovec, n);
 
         j = 0;
         iovec_advance(iov, &j, *idx);
@@ -1000,7 +1000,7 @@ int bus_socket_read_message(sd_bus *bus) {
                                         return -ENOMEM;
                                 }
 
-                                memcpy(f + bus->n_fds, CMSG_DATA(cmsg), n * sizeof(int));
+                                memcpy_safe(f + bus->n_fds, CMSG_DATA(cmsg), n * sizeof(int));
                                 bus->fds = f;
                                 bus->n_fds += n;
                         } else

src/libsystemd/sd-resolve/sd-resolve.c

@@ -219,9 +219,8 @@ static void *serialize_addrinfo(void *p, const struct addrinfo *ai, size_t *leng
 
         memcpy((uint8_t*) p, &s, sizeof(AddrInfoSerialization));
         memcpy((uint8_t*) p + sizeof(AddrInfoSerialization), ai->ai_addr, ai->ai_addrlen);
-
-        if (ai->ai_canonname)
-                memcpy((char*) p + sizeof(AddrInfoSerialization) + ai->ai_addrlen, ai->ai_canonname, cnl);
+        memcpy_safe((char*) p + sizeof(AddrInfoSerialization) + ai->ai_addrlen,
+                    ai->ai_canonname, cnl);
 
         *length += l;
         return (uint8_t*) p + l;