cmd/asm: fix crash triggered by nested #define

A panic was in place for an impossible condition that turned
out to be possible if one used a macro to define a macro.

Another go-fuzz "win".

Fixes #12654.

Change-Id: I0a7bb0f0eabb260c986bf7a2288860c78d8db1af
Reviewed-on: https://go-review.googlesource.com/14777Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/asm/internal/lex/lex_test.go

@@ -226,6 +226,35 @@ var lexTests = []lexTest{
 		),
 		"C.\n",
 	},
+	{
+		"nested #define",
+		lines(
+			"#define A #define B THIS",
+			"A",
+			"B",
+		),
+		"THIS.\n",
+	},
+	{
+		"nested #define with args",
+		lines(
+			"#define A #define B(x) x",
+			"A",
+			"B(THIS)",
+		),
+		"THIS.\n",
+	},
+	/* This one fails. See comment in Slice.Col.
+	{
+		"nested #define with args",
+		lines(
+			"#define A #define B (x) x",
+			"A",
+			"B(THIS)",
+		),
+		"x.\n",
+	},
+	*/
 }
 
 func TestLex(t *testing.T) {

src/cmd/asm/internal/lex/slice.go

@@ -44,8 +44,16 @@ func (s *Slice) Line() int {
 }
 
 func (s *Slice) Col() int {
-	// Col is only called when defining a macro, which can't reach here.
-	panic("cannot happen: slice col")
+	// TODO: Col is only called when defining a macro and all it cares about is increasing
+	// position to discover whether there is a blank before the parenthesis.
+	// We only get here if defining a macro inside a macro.
+	// This imperfect implementation means we cannot tell the difference between
+	//	#define A #define B(x) x
+	// and
+	//	#define A #define B (x) x
+	// The first has definition of B has an argument, the second doesn't. Because we let
+	// text/scanner strip the blanks for us, this is extremely rare, hard to fix, and not worth it.
+	return s.pos
 }
 
 func (s *Slice) SetPos(line int, file string) {