FlacExtractor: add condition for zero-length reads

This improves readability by making clearer that reading no bytes from
the input in readFrames() is an expected case if the buffer is full.

PiperOrigin-RevId: 288711841

library/core/src/main/java/com/google/android/exoplayer2/extractor/flac/FlacExtractor.java

@@ -256,15 +256,18 @@ public final class FlacExtractor implements Extractor {
 
     // Copy more bytes into the buffer.
     int currentLimit = buffer.limit();
-    int bytesRead =
-        input.read(
-            buffer.data, /* offset= */ currentLimit, /* length= */ BUFFER_LENGTH - currentLimit);
-    boolean foundEndOfInput = bytesRead == C.RESULT_END_OF_INPUT;
-    if (!foundEndOfInput) {
-      buffer.setLimit(currentLimit + bytesRead);
-    } else if (buffer.bytesLeft() == 0) {
-      outputSampleMetadata();
-      return Extractor.RESULT_END_OF_INPUT;
+    boolean foundEndOfInput = false;
+    if (currentLimit < BUFFER_LENGTH) {
+      int bytesRead =
+          input.read(
+              buffer.data, /* offset= */ currentLimit, /* length= */ BUFFER_LENGTH - currentLimit);
+      foundEndOfInput = bytesRead == C.RESULT_END_OF_INPUT;
+      if (!foundEndOfInput) {
+        buffer.setLimit(currentLimit + bytesRead);
+      } else if (buffer.bytesLeft() == 0) {
+        outputSampleMetadata();
+        return Extractor.RESULT_END_OF_INPUT;
+      }
     }
 
     // Search for a frame.