s3tests: fix multipart test

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>

s3tests/functional/test_s3.py

@@ -4541,6 +4541,8 @@ def generate_random(size, part_size=5*1024*1024):
         this_part_size = min(left, part_size)
         for y in range(this_part_size / chunk):
             s = s + strpart
+        if this_part_size > len(s):
+            s = s + strpart[0:this_part_size - len(s)]
         yield s
         if (x == size):
             return