Fix various compiler warnings with MSVC

Fixes various compiler warnings found with Microsoft Visual Studio 2015
(and earlier versions).

library/platform.c

diff --git a/library/platform.c b/library/platform.c
index 2591c45..8b336c3 100644
--- a/library/platform.c
+++ b/library/platform.c
@@ -237,7 +237,7 @@
     }
 
     fclose( file );
-    return( n );
+    return( (int)n );
 }
 
 int mbedtls_platform_std_nv_seed_write( unsigned char *buf, size_t buf_len )
@@ -255,7 +255,7 @@
     }
 
     fclose( file );
-    return( n );
+    return( (int)n );
 }
 #endif /* MBEDTLS_PLATFORM_NO_STD_FUNCTIONS */