change palloc to malloc for external indexing

[var77]

Palloc has limit of 1GB (source code reference) , so for large indexes it was failing to allocate a necessary buffer to store the index.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 30.55556% with 25 lines in your changes missing coverage. Please review.	Files with missing lines	Patch %	Lines
src/hnsw/external_index_socket.c	28.57%	21 Missing and 4 partials :warning:

:loudspeaker: Thoughts on this report? Let us know!

[var77]

Covered by https://github.com/lanterndata/lantern/pull/333