ostree/parse-datetime: Ensure tm structs are initialised

Otherwise tm.tm_wday remains uninitialised and gets propagated
elsewhere.

Spotted by Coverity as issue #209265.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1059
Approved by: cgwalters

src/ostree/parse-datetime.y

@@ -1280,8 +1280,8 @@ parse_datetime (struct timespec *result, char const *p,
   time_t Start;
   long int Start_ns;
   struct tm const *tmp;
-  struct tm tm;
-  struct tm tm0;
+  struct tm tm = { 0, };
+  struct tm tm0 = { 0, };
   parser_control pc;
   struct timespec gettime_buffer;
   unsigned char c;