VAULT: Mint To should fail if depositor gets 0 VRT for any amount of ST

[coachchucksol]

There are scenarios, like front running an initial deposit, where a depositor can received 0 VRT given some ST deposit. This should never be the case. This is in addition to min_amount_out.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 77.58%. Comparing base (249a57c) to head (5b3556f).

Files with missing lines	Patch %	Lines
vault_program/src/mint_to.rs	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #147      +/-   ##
==========================================
- Coverage   77.59%   77.58%   -0.01%     
==========================================
  Files          87       87              
  Lines        8903     8911       +8     
==========================================
+ Hits         6908     6914       +6     
- Misses       1995     1997       +2     

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[ebatsell]

This change is pretty harmless, would like to understand if this solves all cases of the frontrunning issue or there's more to be done