[PIPEFS] Reduce garbage generation

The current PIPEFS `read()` implementation seems to produce excessive garbage
with use cases such as write several bytes, read everything, write, read...,
since it everytime throws away an almost empty 8Kb buffer.

This commit addes a fix proposed by @cynecx.

Author: atrosinenko

src/library_pipefs.js

@@ -116,6 +116,14 @@ mergeInto(LibraryManager.library, {
           }
         }
 
+        if (toRemove && toRemove == pipe.buckets.length) {
+          // Do not generate excessive garbage in use cases such as
+          // write several bytes, read everything, write several bytes, read everything...
+          toRemove--;
+          pipe.buckets[toRemove].offset = 0;
+          pipe.buckets[toRemove].roffset = 0;
+        }
+
         while (toRemove--) {
           pipe.buckets.shift();
         }