vt: limit maximum value of params in vt_param_get()

So that the value is clamped to the range [0, 0x7fffffff] and retains
the same value, regardless of whether it's interpreted as a signed or
unsigned integer.

Closes #522

CHANGELOG.md

@@ -103,6 +103,8 @@
   (https://codeberg.org/dnkl/foot/issues/503).
 * Sixels with transparent bottom border being resized below the size
   specified in _”Set Raster Attributes”_.
+* Crash caused by certain CSI sequences with very large parameter
+  values (https://codeberg.org/dnkl/foot/issues/522).
 
 
 ### Security

vt.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <limits.h>
 #include <stdint.h>
 #include <stddef.h>
 
@@ -10,9 +11,17 @@ void vt_from_slave(struct terminal *term, const uint8_t *data, size_t len);
 static inline int
 vt_param_get(const struct terminal *term, size_t idx, int default_value)
 {
+    /*
+     * We zero excess bits in parsed param values. In most cases this will
+     * effectively be a no-op; but it prevents negative returns for edge
+     * cases involving unusually large values.
+     */
+    static_assert(INT_MAX >= 0x7fffffff, "POSIX requires INT_MAX >= 0x7fffffff");
+    const unsigned value_mask = 0x7fffffff;
+
     if (term->vt.params.idx > idx) {
-        int value = term->vt.params.v[idx].value;
+        unsigned value = term->vt.params.v[idx].value & value_mask;
-        return value != 0 ? value : default_value;
+        return value != 0 ? (int)value : default_value;
     }
 
     return default_value;