megvii: Return a char pointer for some stubs instead

`mg_facepp[3]` and `mg_facepp[4]` seem to return version strings, which
occasionally get logged if the return value is non-zero.

Return a dummy string for those functions to avoid crashing in strlen.

Change-Id: Icc5d2ab2f72c480f03e652e3849aceaf4cfb498b

Author: timschumi

megvii/megvii.c

@@ -8,12 +8,16 @@ static int stub_fail() {
     return -1;
 }
 
+static char *stub_fail_str() {
+    return "stub";
+}
+
 void *mg_facepp[] = {
     &stub_fail,
     &stub_fail,
     &stub_fail,
-    &stub_fail,
-    &stub_fail,
+    &stub_fail_str,
+    &stub_fail_str,
     &stub_fail,
     &stub_fail,
     &stub_fail,