Fix possible portability issue

The & 0xFF should not be necessary on platforms with 8-bit chars, but one user
reported having problems with his compiler on such a platform.

library/md5.c

diff --git a/library/md5.c b/library/md5.c
index fc86449..a09bba3 100644
--- a/library/md5.c
+++ b/library/md5.c
@@ -66,12 +66,12 @@
 #endif
 
 #ifndef PUT_UINT32_LE
-#define PUT_UINT32_LE(n,b,i)                            \
-{                                                       \
-    (b)[(i)    ] = (unsigned char) ( (n)       );       \
-    (b)[(i) + 1] = (unsigned char) ( (n) >>  8 );       \
-    (b)[(i) + 2] = (unsigned char) ( (n) >> 16 );       \
-    (b)[(i) + 3] = (unsigned char) ( (n) >> 24 );       \
+#define PUT_UINT32_LE(n,b,i)                                    \
+{                                                               \
+    (b)[(i)    ] = (unsigned char) ( ( (n)       ) & 0xFF );    \
+    (b)[(i) + 1] = (unsigned char) ( ( (n) >>  8 ) & 0xFF );    \
+    (b)[(i) + 2] = (unsigned char) ( ( (n) >> 16 ) & 0xFF );    \
+    (b)[(i) + 3] = (unsigned char) ( ( (n) >> 24 ) & 0xFF );    \
 }
 #endif