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-network/dhcp-option.c

@@ -56,12 +56,7 @@ static int option_append(uint8_t options[], size_t size, size_t *offset,
                 options[*offset] = code;
                 options[*offset + 1] = optlen;
 
-                if (optlen) {
-                        assert(optval);
-
-                        memcpy(&options[*offset + 2], optval, optlen);
-                }
-
+                memcpy_safe(&options[*offset + 2], optval, optlen);
                 *offset += optlen + 2;
 
                 break;

src/libsystemd-network/dhcp6-option.c

@@ -73,8 +73,7 @@ int dhcp6_option_append(uint8_t **buf, size_t *buflen, uint16_t code,
         if (r < 0)
                 return r;
 
-        if (optval)
-                memcpy(*buf, optval, optlen);
+        memcpy_safe(*buf, optval, optlen);
 
         *buf += optlen;
         *buflen -= optlen;

src/libsystemd-network/test-dhcp-option.c

@@ -112,14 +112,9 @@ static DHCPMessage *create_message(uint8_t *options, uint16_t optlen,
         message = malloc0(len);
         assert_se(message);
 
-        if (options && optlen)
-                memcpy(&message->options, options, optlen);
-
-        if (file && filelen <= 128)
-                memcpy(&message->file, file, filelen);
-
-        if (sname && snamelen <= 64)
-                memcpy(&message->sname, sname, snamelen);
+        memcpy_safe(&message->options, options, optlen);
+        memcpy_safe(&message->file, file, filelen);
+        memcpy_safe(&message->sname, sname, snamelen);
 
         return message;
 }