wintime: add a precautionary memset to zero.

Coverity observes that sometimes 'struct tm' can have other fields
(e.g. glibc's tm_gmtoff), so it's as well to make sure we initialise
the whole thing to zero.

windows/wintime.c

@@ -10,6 +10,8 @@ struct tm ltime(void)
     SYSTEMTIME st;
     struct tm tm;
 
+    memset(&tm, 0, sizeof(tm));        /* in case there are any other fields */
+
     GetLocalTime(&st);
     tm.tm_sec=st.wSecond;
     tm.tm_min=st.wMinute;