terminal: handle trailing colon in SGR underline parsing (#11113)

A trailing colon with no following sub-parameter (e.g. "ESC[58:4:m")
leaves the colon separator bit set on the last param without adding
another entry to the params array. When the SGR parser later iterates to
that param (4 = underline) and sees the colon bit, it entered the colon
path which asserted slice.len >= 2, but the slice only had one element.

Replace the assert with a bounds check that treats the malformed
sequence as a default single underline.

Add a regression test reproducing the crash from AFL++ fuzzing
(afl-out/stream/default/crashes/id:000021).

#11109

src/terminal/sgr.zig

@@ -249,7 +249,16 @@ pub const Parser = struct {
                     // Colons are fairly rare in the wild.
                     @branchHint(.unlikely);
 
-                    assert(slice.len >= 2);
+                    // A trailing colon with no following sub-param
+                    // (e.g. "ESC[58:4:m") leaves the colon separator
+                    // bit set on the last param without adding another
+                    // entry, so we can see param 4 with a colon but
+                    // nothing after it.
+                    if (slice.len < 2) {
+                        @branchHint(.cold);
+                        break :underline;
+                    }
+
                     if (self.isColon()) {
                         // Invalid/unknown SGRs are just not very likely.
                         @branchHint(.cold);
@@ -1068,3 +1077,30 @@ test "sgr: kakoune input issue underline, fg, and bg" {
 
     try testing.expect(p.next() == null);
 }
+
+// Fuzz crash: afl-out/stream/default/crashes/id:000021
+// Input "ESC [ 5 8 : 4 : m" produces params [58, 4] with colon
+// separator bits set at indices 0 and 1. The trailing colon causes
+// the second iteration to see param 4 (underline) with a colon,
+// triggering assert(slice.len >= 2) with slice.len == 1.
+test "sgr: underline colon with trailing separator and short slice" {
+    var p: Parser = .{
+        .params = &[_]u16{ 58, 4 },
+        .params_sep = sep: {
+            var list = SepList.initEmpty();
+            list.set(0);
+            list.set(1);
+            break :sep list;
+        },
+    };
+
+    // 58:4 is not a valid underline color (sub-param 4 is not 2 or 5),
+    // so it falls through as unknown.
+    try testing.expect(p.next().? == .unknown);
+
+    // Param 4 with a trailing colon but no sub-param is malformed,
+    // so it also falls through as unknown rather than panicking.
+    try testing.expect(p.next().? == .unknown);
+
+    try testing.expect(p.next() == null);
+}