Initialise struct timezone (#1291)

On musl libc gettimeofday (which is used by time_now) does not
populate the timezone struct passed to it as POSIX says:

> If tzp is not a null pointer, the behavior is unspecified.

tz_minuteswest is later multiplied by 60 which can overflow. When tig
is compiled with integer overflow hardening (as is done on Chimera
Linux) via the clang option -fsanitize=signed-integer-overflow this can
result in tig crashing due to the overflow.

src/main.c

@@ -122,7 +122,7 @@ main_add_changes_commit(struct view *view, enum line_type type, const char *pare
 	struct graph *graph = state->graph;
 	struct commit commit = {{0}};
 	struct timeval now;
-	struct timezone tz;
+	struct timezone tz = {0};
 
 	if (!parent)
 		return true;