fix infinite loop in MemoryPCMStream if stream is playing with loop=true

src/audio/MemoryPCMStream.cpp

@@ -24,12 +24,9 @@ void MemoryPCMStream::close() {
 }
 
 size_t MemoryPCMStream::read(char* dst, size_t bufferSize) {
-    if (!open) {
+    if (!open || buffer.empty()) {
         return PCMStream::ERROR;
     }
-    if (buffer.empty()) {
-        return 0;
-    }
     size_t count = std::min<size_t>(bufferSize, buffer.size());
     std::memcpy(dst, buffer.data(), count);
     buffer.erase(buffer.begin(), buffer.begin() + count);