bisect: check strbuf_getline_lf return when reading terms

get_terms() in builtin/bisect.c and read_bisect_terms() in
bisect.c both read the BISECT_TERMS file but do not check the
strbuf_getline_lf() return values. If the file is truncated
(e.g., a partial write from a crash or disk-full condition),
strbuf_getline_lf returns EOF and the strbuf remains empty.
strbuf_detach then returns an empty string, and the term names
silently become "" instead of the expected "bad"/"good" or
custom terms.

In get_terms(), check for EOF and return -1 on truncation,
matching the existing -1 return for a missing file.

In read_bisect_terms(), die with a descriptive message when a
line cannot be read, consistent with the die_errno for a
non-ENOENT open failure in the same function. Unlike get_terms(),
read_bisect_terms() returns void and uses die() for all error
paths, so the die is the appropriate error handling here.

Pointed out by Coverity.

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

Author: dscho

bisect.c

@@ -1019,10 +1019,12 @@ void read_bisect_terms(char **read_bad, char **read_good)
 			die_errno(_("could not read file '%s'"), filename);
 		}
 	} else {
-		strbuf_getline_lf(&str, fp);
+		if (strbuf_getline_lf(&str, fp) == EOF)
+			die(_("could not read bad term from file '%s'"), filename);
 		free(*read_bad);
 		*read_bad = strbuf_detach(&str, NULL);
-		strbuf_getline_lf(&str, fp);
+		if (strbuf_getline_lf(&str, fp) == EOF)
+			die(_("could not read good term from file '%s'"), filename);
 		free(*read_good);
 		*read_good = strbuf_detach(&str, NULL);
 	}

builtin/bisect.c

@@ -495,9 +495,15 @@ static int get_terms(struct bisect_terms *terms)
 	}
 
 	free_terms(terms);
-	strbuf_getline_lf(&str, fp);
+	if (strbuf_getline_lf(&str, fp) == EOF) {
+		res = -1;
+		goto finish;
+	}
 	terms->term_bad = strbuf_detach(&str, NULL);
-	strbuf_getline_lf(&str, fp);
+	if (strbuf_getline_lf(&str, fp) == EOF) {
+		res = -1;
+		goto finish;
+	}
 	terms->term_good = strbuf_detach(&str, NULL);
 
 finish: