Rewrite of StreamingRingBuffer::safe_read

safe_read was based on FFmpeg's ffurl_read_complete, however when an EOF or an error is encountered, ffurl_read_complete returns the error code rather than the number of bytes read so far.
Rather than wait for FFmpeg to fix it (ref: https://ffmpeg.org/trac/ffmpeg/ticket/2537) ; work around it.
safe_read now will set errno as expected and always return a value >= 0 representing the number of bytes actually read.

Fixes #11263

mythtv/libs/libmythtv/streamingringbuffer.cpp

@@ -101,9 +101,26 @@ long long StreamingRingBuffer::Seek(long long pos, int whence, bool has_lock)
 
 int StreamingRingBuffer::safe_read(void *data, uint sz)
 {
+    uint len = 0;
+
     if (m_context)
-        return ffurl_read_complete(m_context, (unsigned char*)data, sz);
-    return 0;
+    {
+        while (len < sz)
+        {
+            int ret = ffurl_read(m_context, (unsigned char*)data + len, sz - len);
+            if (ret < 0)
+            {
+                if (ret == AVERROR_EOF)
+                {
+                    ateof = true;
+                }
+                errno = ret;
+                break;
+            }
+            len += ret;
+        }
+    }
+    return len;
 }
 
 long long StreamingRingBuffer::GetRealFileSize(void) const