lib: fix infinite loop in __darr_in_vsprintf

vsnprintf returns the number of bytes to write *not including* the
terminating NULL byte. When increasing the capacity of the array, we
should add one more byte, otherwise, if the error length is exactly a
power of 2, the capacity is not increased enough and vsnprintf always
fails.

Signed-off-by: Igor Ryzhov <[email protected]>

lib/darr.c

@@ -77,7 +77,7 @@ char *__darr_in_vsprintf(char **sp, bool concat, const char *fmt, va_list ap)
 	else if ((size_t)len < darr_avail(*sp))
 		_darr_len(*sp) += len;
 	else {
-		darr_ensure_cap(*sp, darr_len(*sp) + (size_t)len);
+		darr_ensure_cap(*sp, darr_len(*sp) + (size_t)len + 1);
 		goto again;
 	}
 	return *sp;