upas: fix warnings

smtp.c:232: warning: comparison with string literal results in unspecified behavior
smtp.c:244: warning: comparison with string literal results in unspecified behavior
marshal.c:1179: warning: variable ‘err’ set but not used

LGTM=rsc
R=rsc
https://codereview.appspot.com/93290043
diff --git a/src/cmd/upas/marshal/marshal.c b/src/cmd/upas/marshal/marshal.c
index 7cef182..8ebd395 100644
--- a/src/cmd/upas/marshal/marshal.c
+++ b/src/cmd/upas/marshal/marshal.c
@@ -1183,7 +1183,7 @@
 		err = w->msg;
 	if(pid >= 0 && (w=procwait(pid)) && w->msg[0])
 		err = w->msg;
-	return nil;
+	return err;
 }
 
 int
diff --git a/src/cmd/upas/smtp/smtp.c b/src/cmd/upas/smtp/smtp.c
index 893f578..62ef865 100644
--- a/src/cmd/upas/smtp/smtp.c
+++ b/src/cmd/upas/smtp/smtp.c
@@ -229,7 +229,7 @@
 	for(i = 0; i < argc; i++){
 		if((trv = rcptto(argv[i])) != 0){
 			/* remember worst error */
-			if(rv != Giveup)
+			if(strcmp(rv, Giveup) != 0)
 				rv = trv;
 			errs[rcvrs] = strdup(s_to_c(reply));
 			removenewline(errs[rcvrs]);
@@ -241,7 +241,7 @@
 	}
 
 	/* if no ok rcvrs or worst error is retry, give up */
-	if(ok == 0 || rv == Retry)
+	if(ok == 0 || strcmp(rv, Retry) == 0)
 		goto error;
 
 	if(ping){