eina: Provide actual len (after possible truncation) to eina_stringshare_add_length

Summary:
vsnprintf() returns "The number of characters that would have been written if n had been sufficiently large",
so if this is larger than the requested length, we pass the requested length to eina_stringshare_add_length,
to avoid reading uninitialised memory.

Fix T6903

Reviewers: zmike

Subscribers: cedric

Maniphest Tasks: T6903

Differential Revision: https://phab.enlightenment.org/D6011

src/lib/eina/eina_stringshare.c

@@ -679,6 +679,8 @@ eina_stringshare_nprintf(unsigned int len, const char *fmt, ...)
 
    if (size < 1)
      return "";
+   if (size > len)
+     size = len;
 
    return eina_stringshare_add_length(tmp, size);
 }