Fix destination offset in Vec_Append when element_size > 1

(cherry picked from commit 54d665b)

Quake/common.c

@@ -189,7 +189,7 @@ void Vec_Append (void **pvec, size_t element_size, const void *data, size_t coun
 	if (!count)
 		return;
 	Vec_Grow (pvec, element_size, count);
-	memcpy ((byte *)*pvec + VEC_HEADER(*pvec).size, data, count * element_size);
+	memcpy ((byte *)*pvec + VEC_HEADER(*pvec).size * element_size, data, count * element_size);
 	VEC_HEADER(*pvec).size += count;
 }