Fail the coverage job when the report holds no records

The gcc capture failure left build/lcov.info absent, but the Codecov
and artifact steps run with if: always(), so the job still uploaded,
logged "No coverage reports found" as a non-fatal warning, and
published a stub artifact. A report that exists but carries no line
records would pass even that much in silence.

Assert at least one DA record on both legs before listing, so an empty
or missing report stops the job where it is produced instead of
surfacing later as a quiet gap in Codecov.

Author: imakris

.github/workflows/coverage.yml

@@ -91,6 +91,7 @@ jobs:
             "*/tools/mexce_protect_lib.h" \
             "*/tools/mexce_protect.cpp" \
             -o lcov.info
+          grep -q '^DA:' lcov.info || { echo "lcov.info holds no coverage records"; exit 1; }
           lcov --list lcov.info
 
       # Clang/LLVM: export every instrumented executable, then keep production files.
@@ -125,6 +126,7 @@ jobs:
             "*/tools/mexce_protect_lib.h" \
             "*/tools/mexce_protect.cpp" \
             -o lcov.info
+          grep -q '^DA:' lcov.info || { echo "lcov.info holds no coverage records"; exit 1; }
           lcov --list lcov.info
 
       - name: Upload coverage to Codecov