liblog: logprint: printable nul

(cherry pick from commit faa92e99151ccec81073cb7ded2d49458468a02d)

Change-Id: I57d1f86ce040ad196c558bb72ac0464af793c214
diff --git a/liblog/logprint.c b/liblog/logprint.c
index 97e28fd..b6dba2e 100644
--- a/liblog/logprint.c
+++ b/liblog/logprint.c
@@ -819,7 +819,7 @@
     uint32_t utf32;
 
     if ((first_char & 0x80) == 0) { /* ASCII */
-        return 1;
+        return first_char ? 1 : -1;
     }
 
     /*