avoid_negative_signed_int.diff

diff --git a/libec_crcs_original.py b/libec_crcs.py
index a80c823..518c737 100644
--- a/libec_crcs_original.py
+++ b/libec_crcs.py
@@ -13,7 +13,15 @@ libec_crc_alt = getattr(libec, 'liberasurecode_crc32_alt', libec.crc32)
 
 
 def crc_alt(data):
-       return ctypes.c_uint(libec_crc_alt(0, ctypes.c_char_p(data), len(data))).value
+        return ctypes.c_uint(libec_crc_alt(0, ctypes.c_char_p(data), len(data))).value
+
+
+def crc_zlib(data):
+        zlib_crc = zlib.crc32(data)
+        if zlib_crc < 0:
+            zlib_crc = zlib_crc & 0xffffffff
+        return zlib_crc
+
 
 for frag in sys.argv[1:]:
        with open(frag, 'rb') as fp:
@@ -29,6 +37,6 @@ for frag in sys.argv[1:]:
        ).lstrip().format(
                frag,
                struct.unpack('I', header[67:71])[0],
-               zlib.crc32(header[:59]),
+               crc_zlib(header[:59]),
                crc_alt(header[:59]),
        ))