Fix char signedness issue in _crc32c_sw_slicing_by_8()

Fix `_crc32c_sw_slicing_by_8()` to use `unsigned char` for `p_buf`,
to fix incorrect results on platforms with signed `char` such as SPARC.
The code has been casting `unsigned char *` to `char *` for no apparent
reason, and this broke the bitshifts in the big endian blocks.

Particularly,

    crc ^= *(p_buf++) << 16

would be XOR-ed against `0xffee0000` rather than `0x00ee0000`.

Fixes #43

crc32c_sw.c

@@ -490,7 +490,7 @@ const uint32_t crc_tableil8_o88[256] =
 
 uint32_t _crc32c_sw_slicing_by_8(uint32_t crc, unsigned const char* data, unsigned long length)
 {
-	const char* p_buf = (const char*) data;
+	unsigned const char* p_buf = data;
 	size_t initial_bytes = (sizeof(uint32_t) - (intptr_t)p_buf) & (sizeof(uint32_t) - 1);
 	size_t li;
 	size_t running_length;
@@ -554,4 +554,4 @@ uint32_t _crc32c_sw_slicing_by_8(uint32_t crc, unsigned const char* data, unsign
 	}
 
 	return crc;
-}
+}