Fix alarm(0) failure on mingw32

A new test for mbedtls_timing_alarm(0) was introduced in PR 1136, which also
fixed it on Unix. Apparently test results on MinGW were not checked at that
point, so we missed that this new test was also failing on this platform.
diff --git a/ChangeLog b/ChangeLog
index af43a59..6aabad4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -43,7 +43,7 @@
    * Fix word size check in in pk.c to not depend on MBEDTLS_HAVE_INT64.
    * Fix crash when calling mbedtls_ssl_cache_free() twice. Found by
      MilenkoMitrovic, #1104
-   * Fix mbedtls_timing_alarm(0) on Unix.
+   * Fix mbedtls_timing_alarm(0) on Unix and MinGW.
    * Fix use of uninitialized memory in mbedtls_timing_get_timer when reset=1.
    * Fix issue in RSA key generation program programs/x509/rsa_genkey
      where the failure of CTR DRBG initialization lead to freeing an
diff --git a/library/timing.c b/library/timing.c
index 1489383..83d0287 100644
--- a/library/timing.c
+++ b/library/timing.c
@@ -268,6 +268,14 @@
 {
     DWORD ThreadId;
 
+    if( seconds == 0 )
+    {
+        /* No need to create a thread for this simple case.
+         * Also, this shorcut is more reliable at least on MinGW32 */
+        alarmed = 1;
+        return;
+    }
+
     alarmed = 0;
     alarmMs = seconds * 1000;
     CloseHandle( CreateThread( NULL, 0, TimerProc, NULL, 0, &ThreadId ) );