Prevent UB on memcpy and floating point conversions

- add `memcpy_no_ub` that accepts null pointers for 0 count
- prevent 0 length allocation in `js_worker_postMessage`
- use safer test for `int` value in `JS_NewFloat64`,
  `JS_ToArrayLengthFree` and `js_typed_array_indexOf`

cutils.h

@@ -26,6 +26,7 @@
 #define CUTILS_H
 
 #include <stdlib.h>
+#include <string.h>
 #include <inttypes.h>
 
 #define likely(x)       __builtin_expect(!!(x), 1)
@@ -64,6 +65,12 @@ char *pstrcat(char *buf, int buf_size, const char *s);
 int strstart(const char *str, const char *val, const char **ptr);
 int has_suffix(const char *str, const char *suffix);
 
+/* Prevent UB when n == 0 and (src == NULL or dest == NULL) */
+static inline void memcpy_no_ub(void *dest, const void *src, size_t n) {
+    if (n)
+        memcpy(dest, src, n);
+}
+
 static inline int max_int(int a, int b)
 {
     if (a > b)