StringIO#pread: handle 0 length like IO#pread (#67)

Fix: #66

If length is 0, IO#pread don't even try to read the IO, it simply return
the buffer untouched if there is one or a new empty buffer otherwise.

It also doesn't validate the offset when length is 0.

cc @jdelStrother @kou

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

Author: casperisfine

ext/stringio/stringio.c

@@ -1602,6 +1602,13 @@ strio_pread(int argc, VALUE *argv, VALUE self)
         rb_raise(rb_eArgError, "negative string size (or size too big): %" PRIsVALUE, rb_len);
     }
 
+    if (len == 0) {
+        if (NIL_P(rb_buf)) {
+            return rb_str_new("", 0);
+        }
+        return rb_buf;
+    }
+
     if (offset < 0) {
         rb_syserr_fail_str(EINVAL, rb_sprintf("pread: Invalid offset argument: %" PRIsVALUE, rb_offset));
     }

test/stringio/test_stringio.rb

@@ -750,6 +750,13 @@ def test_pread
     assert_raise(EOFError) { f.pread(1, 5) }
     assert_raise(ArgumentError) { f.pread(-1, 0) }
     assert_raise(Errno::EINVAL) { f.pread(3, -1) }
+
+    assert_equal "".b, StringIO.new("").pread(0, 0)
+    assert_equal "".b, StringIO.new("").pread(0, -10)
+
+    buf = "stale".b
+    assert_equal "stale".b, StringIO.new("").pread(0, 0, buf)
+    assert_equal "stale".b, buf
   end
 
   def test_size