Replace malloc with calloc

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

library/pkparse.c

@@ -57,7 +57,7 @@
 #include "mbedtls/platform.h"
 #else
 #include <stdlib.h>
-#define mbedtls_malloc     malloc
+#define mbedtls_calloc    calloc
 #define mbedtls_free       free
 #endif
 
@@ -93,7 +93,7 @@ int mbedtls_pk_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( MBEDTLS_ERR_PK_MALLOC_FAILED );