os: fix file.read() (#15861)

vlib/os/file.c.v

@@ -237,11 +237,18 @@ pub fn (f &File) read(mut buf []u8) !int {
 	if buf.len == 0 {
 		return IError(Eof{})
 	}
-	nbytes := fread(buf.data, 1, buf.len, f.cfile) or {
-		return IError(NotExpected{
-			cause: 'unexpected error from fread'
-			code: -1
-		})
+	nbytes := int(C.fread(buf.data, 1, buf.len, f.cfile))
+	// if no bytes were read, check for errors and end-of-file.
+	if nbytes <= 0 {
+		if C.feof(f.cfile) != 0 {
+			return IError(Eof{})
+		}
+		if C.ferror(f.cfile) != 0 {
+			return IError(NotExpected{
+				cause: 'unexpected error from fread'
+				code: -1
+			})
+		}
 	}
 	return nbytes
 }