pkg/ipc: add fallback signal only if normal signal is not enabled

It's possible to get no signal from normal coverage due to dedup,
in that case we don't want to add fallback coverage
because it can lead to corpus bloat.

pkg/ipc/ipc.go

@@ -311,7 +311,7 @@ func (env *Env) Exec(opts *ExecOpts, p *prog.Prog) (output []byte, info []CallIn
 
 	if env.out != nil {
 		info, err0 = env.readOutCoverage(p)
-		if info != nil {
+		if info != nil && env.config.Flags&FlagSignal == 0 {
 			addFallbackSignal(p, info)
 		}
 	}