cmd/asm: fix crash on bad symbol for TEXT

Was missing a check in validSymbol.

Fixes #23580.

Can wait for go1.11. Probably safe but the crash is only for
invalid input, so not worth the risk.

Change-Id: I51f88c5be35a8880536147d1fe5c5dd6798c29de
Reviewed-on: https://go-review.googlesource.com/90398Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/asm/internal/asm/asm.go

@@ -71,7 +71,7 @@ func (p *Parser) append(prog *obj.Prog, cond string, doLabel bool) {
 
 // validSymbol checks that addr represents a valid name for a pseudo-op.
 func (p *Parser) validSymbol(pseudo string, addr *obj.Addr, offsetOk bool) bool {
-	if addr.Name != obj.NAME_EXTERN && addr.Name != obj.NAME_STATIC || addr.Scale != 0 || addr.Reg != 0 {
+	if addr.Sym == nil || addr.Name != obj.NAME_EXTERN && addr.Name != obj.NAME_STATIC || addr.Scale != 0 || addr.Reg != 0 {
 		p.errorf("%s symbol %q must be a symbol(SB)", pseudo, symbolName(addr))
 		return false
 	}

src/cmd/asm/internal/asm/pseudo_test.go

@@ -36,13 +36,16 @@ func TestErroneous(t *testing.T) {
 		{"TEXT", "$\"foo\", 0, $1", "TEXT symbol \"<erroneous symbol>\" must be a symbol(SB)"},
 		{"TEXT", "$0É:0, 0, $1", "expected end of operand, found É"}, // Issue #12467.
 		{"TEXT", "$:0:(SB, 0, $1", "expected '(', found 0"},          // Issue 12468.
+		{"TEXT", "@B(SB),0,$0", "expected '(', found B"},             // Issue 23580.
 		{"FUNCDATA", "", "expect two operands for FUNCDATA"},
 		{"FUNCDATA", "(SB ", "expect two operands for FUNCDATA"},
 		{"DATA", "", "expect two operands for DATA"},
 		{"DATA", "0", "expect two operands for DATA"},
 		{"DATA", "(0), 1", "expect /size for DATA argument"},
+		{"DATA", "@B(SB)/4,0", "expected '(', found B"}, // Issue 23580.
 		{"GLOBL", "", "expect two or three operands for GLOBL"},
 		{"GLOBL", "0,1", "GLOBL symbol \"<erroneous symbol>\" must be a symbol(SB)"},
+		{"GLOBL", "@B(SB), 0", "expected '(', found B"}, // Issue 23580.
 		{"PCDATA", "", "expect two operands for PCDATA"},
 		{"PCDATA", "1", "expect two operands for PCDATA"},
 	}