Apply longjmp(, 0) fix to emscripten longjmp as well (#21577)

Fixes: #21486

Author: yamt

system/lib/compiler-rt/emscripten_setjmp.c

@@ -67,6 +67,12 @@ uint32_t testSetjmp(uintptr_t id, TableEntry* table, uint32_t size) {
 #include "emscripten_internal.h"
 
 void emscripten_longjmp(uintptr_t env, int val) {
+  // C standard:
+  //   The longjmp function cannot cause the setjmp macro to return
+  //   the value 0; if val is 0, the setjmp macro returns the value 1.
+  if (val == 0) {
+    val = 1;
+  }
   setThrew(env, val);
   _emscripten_throw_longjmp();
 }

test/core/test_longjmp_zero.c

@@ -5,6 +5,7 @@
  * found in the LICENSE file.
  */
 
+#include <assert.h>
 #include <stdio.h>
 #include <setjmp.h>
 
@@ -13,6 +14,7 @@ int main() {
   jmp_buf b1;
   int val = setjmp(b1);
   if (val) {
+    assert(val == 1);
     printf("success\n");
     return 0;
   }