Timing: fix mbedtls_set_alarm(0) on Unix/POSIX

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

ChangeLog

@@ -12,6 +12,9 @@ Security
      was independently reported by Tim Nordell via e-mail and by Florin Petriuc
      and sjorsdewit on GitHub. Fix proposed by Florin Petriuc in #1022. Fixes #707.
 
+Features
+   * Allow comments in test data files.
+
 Bugfix
    * Fix ssl_parse_record_header() to silently discard invalid DTLS records
      as recommended in RFC 6347 Section 4.1.2.7.
@@ -50,15 +53,13 @@ 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
      and the message digest. Further, allow enabling/disabling of authority
      identifier, subject identifier and basic constraints extensions.
 
-Features
-   * Allow comments in test data files.
-
 = mbed TLS 2.1.9 branch released 2017-08-10
 
 Security

library/timing.c

@@ -310,6 +310,12 @@ void mbedtls_set_alarm( int seconds )
     mbedtls_timing_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. */
+        mbedtls_timing_alarmed = 1;
+    }
 }
 
 #endif /* _WIN32 && !EFIX64 && !EFI32 */