No need to clear the buffer a second time in StringIO#sysread and #read_nonblock

* StringIO#read already clears it when returning nil.

Author: eregon

lib/truffle/stringio.rb

@@ -526,7 +526,6 @@ def sysread(length = nil, buffer = '')
     str = read(length, buffer)
 
     if str.nil?
-      buffer.clear
       raise EOFError, 'end of file reached'
     end
 
@@ -539,7 +538,6 @@ def read_nonblock(length, buffer = '', exception: true)
     str = read(length, buffer)
 
     if exception and str.nil?
-      buffer.clear
       raise EOFError, 'end of file reached'
     end