Fix for b/4173948 - video & audio hang after multiple seeks

Resolves the problem of the file offset being set to a large
value if readAt returns an error instead of number of bytes
read.  I haven't been able to repro the failure yet since
putting in this fix, so we still need to confirm that the player
recovers correctly when the server error occurs.

Change-Id: I963191144835a89310ad1b0ea0863dd536dff564

proprietary/wvm/WVMFileSource.cpp

@@ -50,24 +50,15 @@ void WVMFileSource::Seek(unsigned long long offset)
 
 size_t WVMFileSource::Read(size_t amount, unsigned char *buffer)
 {
-    size_t result = mDataSource->readAt(mOffset, buffer, amount);
+    ssize_t result = mDataSource->readAt(mOffset, buffer, amount);
 
-#if 0
-    // debug code - log packets to files
-    char filename[32];
-    static int counter = 0;
-    sprintf(filename, "/data/wv/buf%d", counter++);
-    FILE *f = fopen(filename, "w");
-    if (!f)
-        LOGE("WVMFileSource: can't open %s", filename);
-    else {
-        fwrite(buffer, amount, 1, f);
-        fclose(f);
+    if (result < 0) {
+        LOGE("mDataSource-readAt returned error %d\n", (int)result );
+        result = 0;
+    } else  {
+        mOffset += result;
     }
-    LOGD("WVMFileSource::Read(%d bytes to buf=%p)", amount, buffer);
-#endif
 
-    mOffset += result;
     return result;
 }