Add ubsan handling for double cast

quickjs.h

@@ -568,6 +568,16 @@ static js_force_inline JSValue JS_NewUint32(JSContext *ctx, uint32_t val)
 JSValue JS_NewBigInt64(JSContext *ctx, int64_t v);
 JSValue JS_NewBigUint64(JSContext *ctx, uint64_t v);
 
+#ifdef STRICT_R_HEADERS
+// QJS tries to cast to int32_t to see if the value can be represented with
+// less memory, but this results in undefined-beheaviour sanitizer errors.
+// As this is intentional, disable the sanitizer for this function.
+__attribute__((no_sanitize("undefined")))
+static inline int32_t try_double_to_int32(double d) {
+    return (int32_t)d;
+}
+#endif
+
 static js_force_inline JSValue JS_NewFloat64(JSContext *ctx, double d)
 {
     JSValue v;
@@ -577,7 +587,11 @@ static js_force_inline JSValue JS_NewFloat64(JSContext *ctx, double d)
         uint64_t u;
     } u, t;
     u.d = d;
+#ifdef STRICT_R_HEADERS
+    val = try_double_to_int32(d);
+#else
     val = (int32_t)d;
+#endif
     t.d = val;
     /* -0 cannot be represented as integer, so we compare the bit
         representation */