Avoiding arithmetic on NULL pointer

Causes a warning with -Wnull-pointer-arithmetic and the code wasn't doing
anything anyway (the ctl call ignores NULLs)
diff --git a/src/opus_encoder.c b/src/opus_encoder.c
index bdbedec..1c5a8b3 100644
--- a/src/opus_encoder.c
+++ b/src/opus_encoder.c
@@ -2036,8 +2036,6 @@
        info.signalType = st->silk_mode.signalType;
        info.offset = st->silk_mode.offset;
        celt_encoder_ctl(celt_enc, CELT_SET_SILK_INFO(&info));
-    } else {
-       celt_encoder_ctl(celt_enc, CELT_SET_SILK_INFO((SILKInfo*)NULL));
     }
 
     /* 5 ms redundant frame for CELT->SILK */