string-util: check for short input in previous_ansi_sequence()

ellipsize_mem() scans backwards for ANSI escape sequences and calls
previous_ansi_sequence(s, t - s, ...) as t walks down toward s. When
t reaches s + 1 the helper is invoked with length == 1 and computes
'length - 2', which wraps to SIZE_MAX - 1.

Follow-up for cb558ab222

src/test/test-string-util.c

@@ -9,6 +9,17 @@
 #include "strv.h"
 #include "tests.h"
 
+TEST(ellipsize_mem_ansi_short) {
+        _cleanup_free_ char *a = ellipsize_mem("X\x1b[m", 4, 1, 50);
+        assert_se(a);
+
+        _cleanup_free_ char *b = ellipsize_mem(" \x1b[A", 4, 1, 0);
+        assert_se(b);
+
+        _cleanup_free_ char *c = ellipsize_mem("\x1b[m", 3, 1, 50);
+        assert_se(c);
+}
+
 TEST(xsprintf) {
         char buf[5];