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/util.h

@@ -104,6 +104,16 @@ static inline void qsort_safe(void *base, size_t nmemb, size_t size, comparison_
         qsort(base, nmemb, size, compar);
 }
 
+/**
+ * Normal memcpy requires src to be nonnull. We do nothing if n is 0.
+ */
+static inline void memcpy_safe(void *dst, const void *src, size_t n) {
+        if (n == 0)
+                return;
+        assert(src);
+        memcpy(dst, src, n);
+}
+
 int on_ac_power(void);
 
 #define memzero(x,l) (memset((x), 0, (l)))