[minip] Fix undefined behavior in mac_addr_copy

There was a possibly unaligned store in mac_addr_copy.
Replace it with a memcpy call that should generate equivalent,
well defined code.

lib/minip/minip-internal.h

@@ -141,6 +141,5 @@ bool net_timer_cancel(net_timer_t *) __NONNULL();
 void net_timer_init(void);
 
 static inline void mac_addr_copy(uint8_t *dest, const uint8_t *src) {
-    *(uint32_t *)dest = *(const uint32_t *)src;
-    *(uint16_t *)(dest + 4) = *(const uint16_t *)(src + 4);
+    memcpy(dest, src, 6);
 }