inherit_tracking(): defensive programming

CodeQL points out that `branch_get()` can return NULL values.

Note that the error path in this instance calls `BUG()`, not `die()`,
for two reasons:

1. The code lives in `libgit.a` and calling `die()` from within those
   library functions is a bad practice that needs to be reduced, rather
   than increased.

2. The `inherit_tracking()` function really should only be called with
   the name of an existing branch, therefore a `NULL` return value would
   indeed constitute a bug in Git's code.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

branch.c

@@ -224,6 +224,8 @@ static int inherit_tracking(struct tracking *tracking, const char *orig_ref)
 	skip_prefix(orig_ref, "refs/heads/", &bare_ref);
 
 	branch = branch_get(bare_ref);
+	if (!branch)
+		BUG("could not get branch for '%s", bare_ref);
 	if (!branch->remote_name) {
 		warning(_("asked to inherit tracking from '%s', but no remote is set"),
 			bare_ref);