Add header guard around malloc(0) returning NULL implementation

Make it safe to import the config multiple times without having
multiple definition errors.

(This prevents errors in the fuzzers in a later patch.)

Signed-off-by: Daniel Axtens <dja@axtens.net>

tests/configs/config-wrapper-malloc-0-null.h

@@ -21,6 +21,8 @@
 #include "mbedtls/mbedtls_config.h"
 
 #include <stdlib.h>
+
+#ifndef MBEDTLS_PLATFORM_STD_CALLOC
 static inline void *custom_calloc( size_t nmemb, size_t size )
 {
     if( nmemb == 0 || size == 0 )
@@ -30,3 +32,4 @@ static inline void *custom_calloc( size_t nmemb, size_t size )
 
 #define MBEDTLS_PLATFORM_MEMORY
 #define MBEDTLS_PLATFORM_STD_CALLOC custom_calloc
+#endif