MALLOC_DEBUG: enable the option libc.debug.malloc = 10

Fix the compile warning to let the libc.debug.malloc=10 works well
Due to unsuitable value comparison, which cause compiler optimize the
code of comparing two digits.

Change-Id: I0bedd596c9ca2ba308fb008da20ecb328d8548f5
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Author: liu chuansheng <chuansheng.liu@intel.com>

libc/bionic/malloc_debug_leak.c

@@ -263,7 +263,7 @@ int get_backtrace(intptr_t* addrs, size_t max_entries)
 // =============================================================================
 
 #define CHK_FILL_FREE           0xef
-#define CHK_SENTINEL_VALUE      0xeb
+#define CHK_SENTINEL_VALUE      (char)0xeb
 #define CHK_SENTINEL_HEAD_SIZE  16
 #define CHK_SENTINEL_TAIL_SIZE  16
 #define CHK_OVERHEAD_SIZE       (   CHK_SENTINEL_HEAD_SIZE +    \