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/basic/hexdecoct.c

@@ -27,6 +27,7 @@
 #include "alloc-util.h"
 #include "hexdecoct.h"
 #include "macro.h"
+#include "util.h"
 
 char octchar(int x) {
         return '0' + (x & 7);
@@ -574,7 +575,7 @@ static int base64_append_width(char **prefix, int plen,
         if (!t)
                 return -ENOMEM;
 
-        memcpy(t + plen, sep, slen);
+        memcpy_safe(t + plen, sep, slen);
 
         for (line = 0, s = t + plen + slen, avail = len; line < lines; line++) {
                 int act = MIN(width, avail);