Some isaac_ssed fixes:

1) Check for eof (shouldn't happen, but if it does we'll fall into an
infinite loop).
2) Use fatal instead of assert (will work if NDEBUG is ever defined
and provides better diagnostics).
3) Ignore errors from close since they shouldn't matter.

Closes #3679.

src/rt/rust_util.h

@@ -142,15 +142,18 @@ inline void isaac_seed(rust_kernel* kernel, uint8_t* dest, size_t size)
         (_T("CryptReleaseContext"), CryptReleaseContext(hProv, 0));
 #else
     int fd = open("/dev/urandom", O_RDONLY);
-    assert(fd > 0);
+    if (fd == -1)
+        kernel->fatal("error opening /dev/urandom: %s", strerror(errno));
     size_t amount = 0;
     do {
         ssize_t ret = read(fd, dest+amount, size-amount);
-        assert(ret >= 0);
+        if (ret < 0)
+            kernel->fatal("error reading /dev/urandom: %s", strerror(errno));
+        else if (ret == 0)
+            kernel->fatal("somehow hit eof reading from /dev/urandom");
         amount += (size_t)ret;
     } while (amount < size);
-    int ret = close(fd);
-    assert(ret == 0);
+    (void) close(fd);
 #endif
 }