fix: add input validation to prevent runtime errors in rendering functions

Added comprehensive input validation to render_line_plot and render_markers
to prevent runtime errors when plot_data arrays are unallocated or empty.
This fixes the streamplot_demo crash that was blocking test-coverage CI.

- Check if x/y arrays are allocated before accessing
- Validate array sizes are non-zero
- Ensure x and y arrays have matching sizes
- Early return for invalid data instead of crashing

Fixes test-coverage CI failure caused by streamplot_demo runtime error.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: krystophny

src/fortplot_rendering.f90

@@ -40,6 +40,11 @@ subroutine render_line_plot(backend, plot_data, plot_idx, x_min_t, x_max_t, y_mi
         real(wp), allocatable :: x_scaled(:), y_scaled(:)
         integer :: i, n
 
+        ! Validate input data
+        if (.not. allocated(plot_data%x) .or. .not. allocated(plot_data%y)) return
+        if (size(plot_data%x) == 0 .or. size(plot_data%y) == 0) return
+        if (size(plot_data%x) /= size(plot_data%y)) return
+        
         n = size(plot_data%x)
         allocate(x_scaled(n), y_scaled(n))
 
@@ -74,6 +79,11 @@ subroutine render_markers(backend, plot_data, x_min_t, x_max_t, y_min_t, y_max_t
         if (.not. allocated(plot_data%marker)) return
         if (len_trim(plot_data%marker) == 0) return
 
+        ! Validate input data
+        if (.not. allocated(plot_data%x) .or. .not. allocated(plot_data%y)) return
+        if (size(plot_data%x) == 0 .or. size(plot_data%y) == 0) return
+        if (size(plot_data%x) /= size(plot_data%y)) return
+        
         ! Draw markers
         call backend%color(plot_data%color(1), plot_data%color(2), plot_data%color(3))