vis: handle file truncation more gracefully

If we use the file / virtual memory system as cache (using mmap(2))
and another process truncates the file we are editing, we have a
problem. All we can do is catch the resulting SIGBUS, close the
corresponding window and print a warning message.

To test this use:

 $ dd if=/dev/zero of=TEST bs=8M count=1
 $ vis TEST
   :! echo TRUNCATE > TEST

1 files changed, 8 insertions, 0 deletions

diff --git a/text.c b/text.c
index be9a8f5..5f516bc 100644
--- a/text.c
+++ b/text.c
@@ -1185,6 +1185,14 @@ bool text_modified(Text *txt) {
 	return txt->saved_action != txt->history;
 }
 
+bool text_sigbus(Text *txt, const char *addr) {
+	for (Buffer *buf = txt->buffers; buf; buf = buf->next) {
+		if (buf->type == MMAP && buf->data <= addr && addr < buf->data + buf->size)
+			return true;
+	}
+	return false;
+}
+
 enum TextNewLine text_newline_type(Text *txt){
 	if (!txt->newlines) {
 		txt->newlines = TEXT_NEWLINE_NL; /* default to UNIX style \n new lines */