Hyeon-Bin Jeong:  readsome() need to reset gcount to zero.  This fixes http://llvm.org/bugs/show_bug.cgi?id=14670.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@170703 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/istream b/include/istream
index dc1c52b..3979e14 100644
--- a/include/istream
+++ b/include/istream
@@ -1243,6 +1243,7 @@
 streamsize
 basic_istream<_CharT, _Traits>::readsome(char_type* __s, streamsize __n)
 {
+    __gc_ = 0;
     streamsize __c = this->rdbuf()->in_avail();
     switch (__c)
     {