box-drawing: LIGHT ARC: check for sqrt() failures

Closes #914

CHANGELOG.md

@@ -123,6 +123,8 @@
 * Sixel: large image resizes (triggered by e.g. large repeat counts in
   `DECGRI`) are now truncated instead of ignored.
 * Sixel: a repeat count of 0 in `DECGRI` now emits a single sixel.
+* LIGHT ARC box drawing characters incorrectly rendered
+  platforms (https://codeberg.org/dnkl/foot/issues/914).
 
 
 ### Security

box-drawing.c

@@ -1,6 +1,8 @@
 #include "box-drawing.h"
 
 #include <stdio.h>
+#include <math.h>
+#include <fenv.h>
 #include <errno.h>
 
 #define LOG_MODULE "box-drawing"
@@ -1282,9 +1284,19 @@ draw_box_drawings_light_arc(struct buf *buf, wchar_t wc)
     const int num_samples = height * 16;
 
     for (int i = 0; i < num_samples; i++) {
+        errno = 0;
+        feclearexcept(FE_ALL_EXCEPT);
+
         double y = i / 16.;
         double x = sqrt(a2 * (1. - y * y / b2));
 
+        /* See math_error(7) */
+        if (errno != 0 ||
+            fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW))
+        {
+            continue;
+        }
+
         const int row = round(y);
         const int col = round(x);