cgen: guard coverage counter file creation

[guweigang]

Summary

Guard coverage counter file creation before writing coverage rows.

Root cause

The generated vprint_coverage_stats() opened the counter CSV with fopen(...) and immediately passed the result to fprintf(...). If the coverage directory was not writable or was otherwise invalid, fopen could return NULL, causing a segfault during _vcleanup().

This also fixes a typo in the generated non-Windows timestamp assignment, where ts.tv_nsec was assigned to secs instead of nsecs.

Changes

• Return early from generated vprint_coverage_stats() when the counter file cannot be opened.
• Generate nsecs = ts.tv_nsec; for the monotonic timestamp nanoseconds field.
• Add a C output regression test for the generated guard and timestamp assignment.

Validation

• ./v -o ./v4 ./cmd/v
• ./v4 -cov /private/tmp/vcov_as_file vlib/v/tests/fns/closure_struct_init_cov_regression_test.v
• ./v4 -cov /private/tmp/vcov_ok vlib/v/tests/fns/closure_struct_init_cov_regression_test.v
• ./v4 test vlib/v/gen/c/coutput_test.v
• git diff --check