use size_t for MEM_allocN_len as well as some of its callers

intern/guardedalloc/MEM_guardedalloc.h

@@ -66,7 +66,7 @@ extern "C" {
 	/** Returns the lenght of the allocated memory segment pointed at
 	 * by vmemh. If the pointer was not previously allocated by this
 	 * module, the result is undefined.*/
-	int MEM_allocN_len(void *vmemh);
+	size_t MEM_allocN_len(void *vmemh);
 
 	/**
 	 * Release memory previously allocatred by this module. 

intern/guardedalloc/intern/mallocn.c

@@ -220,7 +220,7 @@ void MEM_set_memory_debug(void)
 	malloc_debug_memset= 1;
 }
 
-int MEM_allocN_len(void *vmemh)
+size_t MEM_allocN_len(void *vmemh)
 {
 	if (vmemh) {
 		MemHead *memh= vmemh;