fix(format): never peel a stacked label that names a macro

Peeling a stacked label whose name is a macro moves it onto a line of
its own, where its expansion decides the line's shape: an empty body
leaves a bare colon behind, a line the parser rejects.  The peel loops
now hold such labels back with the rest of the line, and the crashing
input joins the corpus.

Assisted-by: GLM 5.3

format/testdata/fuzz/FuzzFormatExpansionRoundTrip/4173fd7be1b3f1a3

@@ -0,0 +1,2 @@
+go test fuzz v1
+string("#define M \nA:M:")