objtool: Make BP scratch register warning more robust

If func is NULL, a seg fault can result.

This is a theoretical issue which was found by Coverity, ID: 1492002
("Dereference after null check").

Fixes: c705cec ("objtool: Track original function across branches")
Reported-by: Gustavo A. R. Silva <[email protected]>
Signed-off-by: Josh Poimboeuf <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Link: https://lkml.kernel.org/r/afc628693a37acd287e843bcc5c0430263d93c74.1585761021.git.jpoimboe@redhat.com

Author: jpoimboe

tools/objtool/check.c

@@ -2005,8 +2005,8 @@ static int validate_return(struct symbol *func, struct instruction *insn, struct
 	}
 
 	if (state->bp_scratch) {
-		WARN("%s uses BP as a scratch register",
-		     func->name);
+		WARN_FUNC("BP used as a scratch register",
+			  insn->sec, insn->offset);
 		return 1;
 	}