nestegg: ne_buffer_read: quiet uint64->size_t warning

the buffer length is already a size_t making the offset into it limited

Change-Id: Ibc5352e69049186cc07f7d86117696b11a98a600

third_party/nestegg/README.webm

@@ -21,3 +21,4 @@ nestegg.c|975 col 6| warning: ‘r’ may be used uninitialized in this function
 - ne_read_(string|binary|block): normalize size_t usage
 - ne_parse: normalize size_t usage
 - quiet read related uint64->size_t warnings
+- ne_buffer_read: quiet uint64->size_t warning

third_party/nestegg/src/nestegg.c

@@ -1605,7 +1605,7 @@ ne_buffer_read(void * buffer, size_t length, void * user_data)
   struct sniff_buffer * sb = user_data;
 
   int rv = 1;
-  size_t available = sb->length - sb->offset;
+  size_t available = sb->length - (size_t)sb->offset;
 
   if (available < length)
     return 0;