Fix undefined overflow behavior when converting double to integer (#3629)

Overflows in conversions from floating-point to integer are
undefined behavior in the C99 standard. (Clause 6.3.1.4: "If the
value of the integral part cannot be represented by the integer
type, the behavior is undefined.")

When UBSAN is enabled, this gets reported at `srand()` calls. (The
random seed is usually initialized using the date port API, which
represents dates as `double`s. But `srand` takes an `unsigned int`.
A simple cast from `double` to `unsigned` becomes undefined
behavior if the value is too large. And "now" is too large
nowadays. So, effectively, all executions start with an undefined
behavior.)

This patch fixes this by casting the floating-point value of the
date to an integer through a union.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

docs/03.API-EXAMPLE.md

@@ -1084,7 +1084,8 @@ int
 main (void)
 {
   /* Initialize srand value */
-  srand ((unsigned) jerry_port_get_current_time ());
+  union { double d; unsigned u; } now = { .d = jerry_port_get_current_time () };
+  srand (now.u);
 
   /* Generate a random number, and print it */
   const jerry_char_t script[] = "var a = Math.random (); print(a)";