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/nspawn/nspawn.c

@@ -2580,12 +2580,10 @@ static int inner_child(
 
                 /* Automatically search for the init system */
 
-                m = 1 + strv_length(arg_parameters);
-                a = newa(char*, m + 1);
-                if (strv_isempty(arg_parameters))
-                        a[1] = NULL;
-                else
-                        memcpy(a + 1, arg_parameters, m * sizeof(char*));
+                m = strv_length(arg_parameters);
+                a = newa(char*, m + 2);
+                memcpy_safe(a + 1, arg_parameters, m * sizeof(char*));
+                a[1 + m] = NULL;
 
                 a[0] = (char*) "/usr/lib/systemd/systemd";
                 execve(a[0], a, env_use);