Camera2: Fix CameraParameters parse segfault.

Bug: 18331142
Change-Id: I41feec93f753f97b3e6bf963deb4a42b73406421
diff --git a/camera/CameraParameters.cpp b/camera/CameraParameters.cpp
index 25d632d..e5e4e90 100644
--- a/camera/CameraParameters.cpp
+++ b/camera/CameraParameters.cpp
@@ -488,6 +488,11 @@
     const char* supportedPreviewFormats =
           get(CameraParameters::KEY_SUPPORTED_PREVIEW_FORMATS);
 
+    if (supportedPreviewFormats == NULL) {
+        ALOGW("%s: No supported preview formats.", __FUNCTION__);
+        return;
+    }
+
     String8 fmtStr(supportedPreviewFormats);
     char* prevFmts = fmtStr.lockBuffer(fmtStr.size());