Fix invalid C code found by gcc-4.6.

Change-Id: Ib76086577a8b1a6567d91751e137f550ca64a288
diff --git a/src/modules/audio_processing/agc/main/source/analog_agc.c b/src/modules/audio_processing/agc/main/source/analog_agc.c
index dfb7adc..e52fd66 100644
--- a/src/modules/audio_processing/agc/main/source/analog_agc.c
+++ b/src/modules/audio_processing/agc/main/source/analog_agc.c
@@ -921,8 +921,7 @@
         stt->Rxx16_vectorw32[stt->Rxx16pos] = Rxx16w32;
 
         /* Circular buffer */
-        stt->Rxx16pos = stt->Rxx16pos++;
-        if (stt->Rxx16pos == RXX_BUFFER_LEN)
+        if (++(stt->Rxx16pos) == RXX_BUFFER_LEN)
         {
             stt->Rxx16pos = 0;
         }