Fix constant flow mask maths

Signed-off-by: Paul Elliott <paul.elliott@arm.com>

library/base64.c

diff --git a/library/base64.c b/library/base64.c
index f2ae49b..267dcc2 100644
--- a/library/base64.c
+++ b/library/base64.c
@@ -79,7 +79,9 @@
 #endif
 
     /* Generate bitmask from condition, mask will either be 0xFFFFFFFF or 0 */
-    unsigned char mask =  -( unsigned char )( ( condition | -condition ) >> 7 );
+    unsigned char mask = ( condition | -condition );
+    mask >>= 7;
+    mask = -mask;
 
 #if defined(_MSC_VER)
 #pragma warning( pop )
@@ -102,7 +104,9 @@
 #endif
 
     /* Generate bitmask from condition, mask will either be 0xFFFFFFFF or 0 */
-    uint32_t mask =  -( uint32_t )( ( condition | -condition ) >> 31 );
+    uint32_t mask = ( condition | -condition );
+    mask >>= 31;
+    mask = -mask;
 
 #if defined(_MSC_VER)
 #pragma warning( pop )
@@ -111,7 +115,6 @@
     *dest = ( src & mask ) | ( ( *dest ) & ~mask );
 }
 
-
 /*
  * Constant flow check for equality
 */