fall back to len() if wcswidth returns negative value

borg/platform_posix.pyx

@@ -2,4 +2,9 @@ cdef extern from "wchar.h":
     cdef int wcswidth(const Py_UNICODE *str, size_t n)
 
 def swidth(s):
-    return wcswidth(s, len(s))
+    str_len = len(s)
+    terminal_width = wcswidth(s, str_len)
+    if terminal_width >= 0:
+        return terminal_width
+    else:
+        return str_len