[rubygems/rubygems] Fix TarReader::Entry#read/partial to match File#read and StringIO#read

TarReader is used as an IO object, but doesn't behave the same as other
implementations. These fixes make `read` and `readpartial` conform to the
interface of StringIO and File.

https://github.com/rubygems/rubygems/commit/bba32d7217

lib/rubygems/package/tar_reader/entry.rb

@@ -130,9 +130,10 @@ class Gem::Package::TarReader::Entry
   def read(len = nil)
     check_closed
 
-    return nil if @read >= @header.size
-
     len ||= @header.size - @read
+
+    return nil if len > 0 && @read >= @header.size
+
     max_read = [len, @header.size - @read].min
 
     ret = @io.read max_read
@@ -144,9 +145,10 @@ class Gem::Package::TarReader::Entry
   def readpartial(maxlen = nil, outbuf = "".b)
     check_closed
 
-    raise EOFError if @read >= @header.size
-
     maxlen ||= @header.size - @read
+
+    raise EOFError if maxlen > 0 && @read >= @header.size
+
     max_read = [maxlen, @header.size - @read].min
 
     @io.readpartial(max_read, outbuf)