improve bounds checking for line length validation

[metsw24-max]

Improves bounds checking logic in mod_substitute within Apache HTTP Server

Replaces combined arithmetic validation with step-wise boundary checks

Ensures vb.strlen, len, and replen remain within max_line_length limits

Updates conditions to avoid reliance on aggregated expressions for validation

Applies consistent bounds checking pattern across related code paths

Adjusts checks involving space_left to use safe comparisons

Refines validation of regm[0].rm_so against remaining buffer space

Prevents operations from exceeding configured buffer limits