Timing: fix set_alarm(0) on Unix/POSIX

The POSIX/Unix implementation of set_alarm did not set the
alarmed flag when called with 0, which was inconsistent
with what the documentation implied and with the Windows behavior.

ChangeLog

@@ -43,6 +43,7 @@ Bugfix
    * 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.
 
 Changes
    * Extend cert_write example program by options to set the CRT version

library/timing.c

@@ -318,6 +318,12 @@ void set_alarm( int seconds )
     alarmed = 0;
     signal( SIGALRM, sighandler );
     alarm( seconds );
+    if( seconds == 0 )
+    {
+        /* alarm(0) cancelled any previous pending alarm, but the
+           handler won't fire, so raise the flag straight away. */
+        alarmed = 1;
+    }
 }
 
 void m_sleep( int milliseconds )