Replace malloc with calloc

- platform layer currently broken (not adapted yet)
- memmory_buffer_alloc too

programs/util/pem2der.c

@@ -31,7 +31,7 @@
 #else
 #include <stdio.h>
 #define mbedtls_free       free
-#define mbedtls_malloc     malloc
+#define mbedtls_calloc    calloc
 #define mbedtls_printf     printf
 #endif
 
@@ -136,7 +136,7 @@ static int load_file( const char *path, unsigned char **buf, size_t *n )
     *n = (size_t) size;
 
     if( *n + 1 == 0 ||
-        ( *buf = mbedtls_malloc( *n + 1 ) ) == NULL )
+        ( *buf = mbedtls_calloc( 1, *n + 1 ) ) == NULL )
     {
         fclose( f );
         return( -1 );