fix: colored_contours PDF runtime crash - End of record error in pdf_write_color

[krystophny]

Issue

GitHub Pages deployment is failing because the colored_contours example crashes with:

At line 69 of file ././src/fortplot_pdf_drawing.f90
Fortran runtime error: End of record

Stack Trace

The crash occurs in:

1. pdf_write_color at line 69 in fortplot_pdf_drawing.f90
2. set_generic_vector_color at line 130 in fortplot_vector.f90
3. set_pdf_color at line 107 in fortplot_pdf.f90
4. During default_gaussian_example PDF rendering

Root Cause

The format string '(F0.3,1X,F0.3,1X,F0.3," RG")' in pdf_write_color is failing, likely due to:

• Invalid RGB values (NaN, infinity, negative, or out-of-range)
• Invalid color mapping during filled contour rendering

Impact

🚨 CRITICAL: Blocks GitHub Pages deployment and visual showcase restoration

Solution

Add input validation to pdf_write_color to handle invalid RGB values gracefully.