make sure RAND_MAX is a proper value

src/library.js

@@ -3458,7 +3458,7 @@ LibraryManager.library = {
   rand_r: function(seedp) {
     seedp = seedp|0; 
     var val = 0;
-    val = ((Math_imul({{{ makeGetValueAsm('seedp', 0, 'i32') }}}, 31010991)|0) + 0x676e6177 ) & {{{ cDefine('RAND_MAX') }}};
+    val = ((Math_imul({{{ makeGetValueAsm('seedp', 0, 'i32') }}}, 31010991)|0) + 0x676e6177 ) & {{{ cDefine('RAND_MAX') }}}; // assumes RAND_MAX is in bit mask form (power of 2 minus 1)
     {{{ makeSetValueAsm('seedp', 0, 'val', 'i32') }}};
     return val|0;
   },

tests/test_core.py

@@ -3552,8 +3552,13 @@ ok
   def test_rand(self):
     src = r'''#include <stdlib.h>
 #include <stdio.h>
+#include <assert.h>
 int main()
 {
+    // we need RAND_MAX to be a bitmask (power of 2 minus 1). this assertions guarantees
+    // if RAND_MAX changes the test failure will focus attention on that issue here.
+    assert(RAND_MAX == 0x7fffffff);
+
     srand(0xdeadbeef);
     for(int i = 0; i < 10; ++i)
         printf("%d\n", rand());