WifiHal: Release the lock before function return

Release the lock before function return.

Change-Id: I8b8ea91965d4bb9e589780d3584dec549e17d413
CRs-Fixed: 2848067
diff --git a/qcwcn/wifi_hal/ring_buffer.cpp b/qcwcn/wifi_hal/ring_buffer.cpp
index a568a73..5918923 100644
--- a/qcwcn/wifi_hal/ring_buffer.cpp
+++ b/qcwcn/wifi_hal/ring_buffer.cpp
@@ -491,6 +491,7 @@
                 cur_read_len = rbc->bufs[rbc->rd_buf_no].last_wr_index - rbc->cur_rd_buf_idx;
             } else {
                 ALOGE("Alert: cur_read_len=%u invalid, rd_buf[no=%d rd_idx=%d wr_index=%d]",cur_read_len, rbc->rd_buf_no, rbc->cur_rd_buf_idx, rbc->bufs[rbc->rd_buf_no].last_wr_index);
+                rb_unlock(&rbc->rb_rw_lock);
                 return NULL;
             }
         }