cmd/gc: fix imported and not used error for import .

Fixes issues 6420.

R=ken2
CC=golang-dev
https://golang.org/cl/13703044

src/cmd/gc/lex.c

@@ -2296,7 +2296,7 @@ pkgnotused(int lineno, Strlit *path, char *name)
 		elem++;
 	else
 		elem = path->s;
-	if(strcmp(elem, name) == 0)
+	if(name == nil || strcmp(elem, name) == 0)
 		yyerrorl(lineno, "imported and not used: \"%Z\"", path);
 	else
 		yyerrorl(lineno, "imported and not used: \"%Z\" as %s", path, name);
@@ -2335,7 +2335,7 @@ mkpackage(char* pkgname)
 					// throw away top-level name left over
 					// from previous import . "x"
 					if(s->def->pack != N && !s->def->pack->used && !nsyntaxerrors) {
-						pkgnotused(s->def->pack->lineno, s->def->pack->pkg->path, s->name);
+						pkgnotused(s->def->pack->lineno, s->def->pack->pkg->path, nil);
 						s->def->pack->used = 1;
 					}
 					s->def = N;

test/import1.go

@@ -14,5 +14,6 @@ import bufio "os"	// ERROR "redeclared|redefinition|incompatible" "imported and
 
 import (
 	"fmt"	// GCCGO_ERROR "previous|not used"
-	fmt "math"	// ERROR "redeclared|redefinition|incompatible" "imported and not used"
+	fmt "math"	// ERROR "redeclared|redefinition|incompatible" "imported and not used: \x22math\x22 as fmt"
+	. "math"	// ERROR "imported and not used: \x22math\x22$"
 )