Fixes #1431: Incorrect results from exponentModulo.

[ianacaburian]

Changes:

• Removed incorrect mod op on exponent.
• Handled base case for zero exponent.

Tests:

• Tested against modPow by jsbn
• Tested 100 runs of random hex strings with lengths 300 - 1000 using fast-check: ✓ src/JuceBigInteger.test.ts (1) 1776566ms ✓ JuceBigInteger (1) 1776565ms ✓ exponentModulo (1) 1776565ms
• Double-checked counter examples on Omni Calculator:
  • b=x, e=0, m=y
  • b=3, e=8, m=5
  • b=0x1400000007, e=0x1400000006, m=0x1400000005

Source:

• Find the tests in this project: @ianacaburian/generate-key-file

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