asoc: codecs: add a null pointer check in mbhc driver

Add a missing function pointer null check condition in
mbhc driver to avoid null access during fast
insertion-removal.

Change-Id: I6aa64325cea6f7aa551115969d1e6ba184ec8d14
Signed-off-by: Ramprasad Katkam <katkam@codeaurora.org>

asoc/codecs/wcd-mbhc-legacy.c

@@ -1,4 +1,4 @@
-/* Copyright (c) 2015-2017, The Linux Foundation. All rights reserved.
+/* Copyright (c) 2015-2018, The Linux Foundation. All rights reserved.
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 and
@@ -533,7 +533,8 @@ correct_plug_type:
 					mbhc->hs_detect_work_stop);
 			wcd_enable_curr_micbias(mbhc,
 						WCD_MBHC_EN_NONE);
-			if (mbhc->micbias_enable) {
+			if (mbhc->mbhc_cb->mbhc_micb_ctrl_thr_mic &&
+				mbhc->micbias_enable) {
 				mbhc->mbhc_cb->mbhc_micb_ctrl_thr_mic(
 					mbhc->codec, MIC_BIAS_2, false);
 				if (mbhc->mbhc_cb->set_micbias_value)
@@ -558,7 +559,8 @@ correct_plug_type:
 					mbhc->hs_detect_work_stop);
 			wcd_enable_curr_micbias(mbhc,
 						WCD_MBHC_EN_NONE);
-			if (mbhc->micbias_enable) {
+			if (mbhc->mbhc_cb->mbhc_micb_ctrl_thr_mic &&
+				mbhc->micbias_enable) {
 				mbhc->mbhc_cb->mbhc_micb_ctrl_thr_mic(
 					mbhc->codec, MIC_BIAS_2, false);
 				if (mbhc->mbhc_cb->set_micbias_value)