libmedia: add NULL check before accessing source and destination buffers.

IMemory can return a NULL pointer or the destination buffer for readAt can
be NULL. Check for NULL and report error.

Author:    Shivaprasad Hongal <shongal@codeaurora.org>

Change-Id: I6df9275f2eb9a6e1c2641d084d713b73737d68d7
diff --git a/media/libmedia/IMediaHTTPConnection.cpp b/media/libmedia/IMediaHTTPConnection.cpp
index e4b717b..1bb8d67 100644
--- a/media/libmedia/IMediaHTTPConnection.cpp
+++ b/media/libmedia/IMediaHTTPConnection.cpp
@@ -124,6 +124,14 @@
             ALOGE("got %zu, but memory has %zu", len, mMemory->size());
             return ERROR_OUT_OF_RANGE;
         }
+        if(buffer == NULL) {
+           ALOGE("readAt got a NULL buffer");
+           return UNKNOWN_ERROR;
+        }
+        if (mMemory->pointer() == NULL) {
+           ALOGE("readAt got a NULL mMemory->pointer()");
+           return UNKNOWN_ERROR;
+        }
 
         memcpy(buffer, mMemory->pointer(), len);