model loading error handling in denoise.c

[orvitpng]

This patch improves RNN model loading functions by:

• Checking allocation return values before use
• Validating file handles
• Rejecting overly large file sizes (i.e., > INT_MAX)
• Handling malloc(0) explicitly

Functionality should remain the same, but this removes undefined or implementation-defined behaviors.

[jmvalin]

Hi, in general, please make sure that everything is strictly C90-compliant. I don't know that the rest of RNNoise is currently all fully C90, but I'm at least trying to get there.