bisect: check get_terms return at all call sites

Six callers of get_terms() silently discard its return value. When
get_terms fails (missing or truncated BISECT_TERMS file), the term
strings remain NULL or empty, causing confusing downstream
behavior: commands like "bisect next" or "bisect run" proceed with
empty term strings, producing nonsensical ref names (refs/bisect/
with no suffix) and misleading error messages.

Add checks at each call site so that a failed get_terms produces a
clear "no terms defined" error, matching the pattern already used
in bisect_terms() at line 512. The check tests the term pointers
rather than the return value because some callers (bisect skip,
legacy bad/good) call set_terms before get_terms, and the
set_terms values should survive a get_terms failure.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

builtin/bisect.c

@@ -1054,6 +1054,8 @@ static int process_replay_line(struct bisect_terms *terms, struct strbuf *line)
 	*word_end = '\0'; /* NUL-terminate the word */
 
 	get_terms(terms);
+	if (!terms->term_bad || !terms->term_good)
+		return error(_("no terms defined"));
 	if (check_and_set_terms(terms, p))
 		return -1;
 
@@ -1380,6 +1382,8 @@ static int cmd_bisect__next(int argc, const char **argv UNUSED, const char *pref
 		return error(_("'%s' requires 0 arguments"),
 			     "git bisect next");
 	get_terms(&terms);
+	if (!terms.term_bad || !terms.term_good)
+		return error(_("no terms defined"));
 	res = bisect_next(&terms, prefix);
 	free_terms(&terms);
 	return res;
@@ -1414,6 +1418,8 @@ static int cmd_bisect__skip(int argc, const char **argv, const char *prefix UNUS
 
 	set_terms(&terms, "bad", "good");
 	get_terms(&terms);
+	if (!terms.term_bad || !terms.term_good)
+		return error(_("no terms defined"));
 	res = bisect_skip(&terms, argc, argv);
 	free_terms(&terms);
 	return res;
@@ -1426,6 +1432,8 @@ static int cmd_bisect__visualize(int argc, const char **argv, const char *prefix
 	struct bisect_terms terms = { 0 };
 
 	get_terms(&terms);
+	if (!terms.term_bad || !terms.term_good)
+		return error(_("no terms defined"));
 	res = bisect_visualize(&terms, argc, argv);
 	free_terms(&terms);
 	return res;
@@ -1440,6 +1448,8 @@ static int cmd_bisect__run(int argc, const char **argv, const char *prefix UNUSE
 	if (!argc)
 		return error(_("'%s' failed: no command provided."), "git bisect run");
 	get_terms(&terms);
+	if (!terms.term_bad || !terms.term_good)
+		return error(_("no terms defined"));
 	res = bisect_run(&terms, argc, argv);
 	free_terms(&terms);
 	return res;
@@ -1479,6 +1489,8 @@ int cmd_bisect(int argc,
 
 		set_terms(&terms, "bad", "good");
 		get_terms(&terms);
+		if (!terms.term_bad || !terms.term_good)
+			return error(_("no terms defined"));
 		if (check_and_set_terms(&terms, argv[0]) ||
 		    !one_of(argv[0], terms.term_good, terms.term_bad, NULL))
 			usage_msg_optf(_("unknown command: '%s'"), git_bisect_usage,