cgen,checker: support simple voidptr casts in consts, without delaying the initialisation to _vinit (#15996)

vlib/dl/dl.v

@@ -4,6 +4,8 @@ pub const version = 1
 
 pub const dl_ext = get_shared_library_extension()
 
+pub const rtld_next = voidptr(-1)
+
 // get_shared_library_extension returns the platform dependent shared library extension
 // i.e. .dll on windows, .so on most unixes, .dylib on macos.
 [inline]

vlib/dl/dl_nix.c.v

@@ -38,5 +38,9 @@ pub fn sym(handle voidptr, symbol string) voidptr {
 // that occurred from a call to one of the `dl` functions, since the last
 // call to dlerror()
 pub fn dlerror() string {
-	return unsafe { cstring_to_vstring(C.dlerror()) }
+	sptr := C.dlerror()
+	if sptr == unsafe { nil } {
+		return ''
+	}
+	return unsafe { cstring_to_vstring(sptr) }
 }

vlib/dl/rtld_next_test.v

@@ -0,0 +1,14 @@
+import dl
+
+type RealOpen = fn (charptr, int, int) int
+
+fn test_rtld_next() {
+	$if windows {
+		println('skipping test_rtld_next on windows')
+		return
+	}
+	real_open := RealOpen((dl.sym(dl.rtld_next, 'open')))
+	println(ptr_str(real_open))
+	assert real_open != unsafe { nil }
+	assert dl.dlerror() == ''
+}