fix: guard silent chat trim to avoid OOB write

[mytsvy]

Problem

When a silent chat trigger is sent alone, pszMessage becomes empty (V_strlen == 0), leading to a potential out-of-bounds (OOB) write via pszMessage[len - 1] = 0.

Solution

Added a length check inside the bSilent block to ensure the trailing character is only removed when len > 0.

Impact & Testing

• Risk: Low risk; safely handles the empty message edge case.
• Testing: Manually tested with single-character silent triggers (! and /). Works perfectly without any issues, and normal commands continue to function as expected.