Fix 'comparison is always false' warning on ARM/unsigned char platforms

[wdfk-prog]

Title

Fix type limit warning in linenoiseEditFeed on ARM builds

Description

I encountered a compilation warning when cross-compiling for AArch64 (ARMv8) using GCC with -Wextra enabled:

warning: comparison is always false due to limited range of data type [-Wtype-limits]
if (c < 0) return NULL;

Reason:
On ARM platforms, the char type is unsigned by default (0 to 255). Therefore, the check c < 0 never evaluates to true, effectively ignoring error codes returned by completeLine().

Fix:
Explicitly cast c to (signed char) in the conditional check. This ensures that negative return values are correctly identified regardless of the platform's default char signedness.

Verification

• Verified that the warning disappears after the fix on AArch64 cross-compilation.
• Verified that the code still compiles correctly on x86 environments.