randomizer: do not pass NULL to memcpy()

When switching playback order from NORMAL to RANDOM, the current playlist items are added to the randomizer.

However, if the playlist is empty, the items vector is empty, and its `.data field is NULL. This causes the following error in ASAN:

../../src/playlist/randomizer.c:426:10: runtime error: null pointer passed as argument 2, which is declared to never be null

If the playlist size is 0, then do not call randomizer_Add() at all.

include/vlc_vector.h

@@ -20,6 +20,7 @@
 #ifndef VLC_VECTOR_H
 #define VLC_VECTOR_H
 
+#include <assert.h>
 #include <stdbool.h>
 #include <stddef.h>
 
@@ -332,6 +333,7 @@ vlc_vector_growsize_(size_t value)
 
 #define vlc_vector_push_all_internal_(pv, items, count) \
 ( \
+    assert(count), \
     vlc_vector_check_same_ptr_type_((pv)->data, items), \
     vlc_vector_reserve(pv, (pv)->size + (count)) && \
     ( \
@@ -359,6 +361,7 @@ vlc_vector_growsize_(size_t value)
 
 #define vlc_vector_insert_hole_internal_(pv, index, count) \
 ( \
+    assert(count), \
     vlc_vector_reserve(pv, (pv)->size + (count)) && \
     ( \
         (index) == (pv)->size || \
@@ -467,6 +470,7 @@ vlc_vector_rotate_array_right_(char *array, size_t len, size_t distance)
 static inline void
 vlc_vector_move_(char *array, size_t index, size_t count, size_t target)
 {
+    assert(count);
     if (index < target)
         vlc_vector_rotate_array_left_(&array[index], target - index + count,
                                       count);
@@ -529,6 +533,7 @@ vlc_vector_move_(char *array, size_t index, size_t count, size_t target)
 
 #define vlc_vector_remove_slice_noshrink_internal_(pv, index, count) \
     do { \
+        assert(count); \
         if ((index) + (count) < (pv)->size) \
             memmove(&(pv)->data[index], \
                     &(pv)->data[(index) + (count)], \