Mini-refactoring prompted by https://phabricator.fb.com/D1770460

Summary: I got obliged into this one :o).

Test Plan: runtests

Reviewed By: meyering@fb.com

Subscribers: njormrod, folly-diffs@

FB internal diff: D1772328

Signature: t1:1772328:1420744114:2b81e46d77800d247293cb667bbef86e9d6d4755

folly/FBString.h

@@ -2322,16 +2322,15 @@ std::basic_istream<
       n = str.max_size();
     }
     str.erase();
-    auto got = is.rdbuf()->sgetc();
-    for (; extracted != size_t(n) && got != T::eof() &&
-           !isspace(got); ++extracted) {
-      // Whew. We get to store this guy
-      str.push_back(got);
-      got = is.rdbuf()->snextc();
-    }
+    for (auto got = is.rdbuf()->sgetc(); extracted != size_t(n); ++extracted) {
       if (got == T::eof()) {
         err |= __ios_base::eofbit;
         is.width(0);
+        break;
+      }
+      if (isspace(got)) break;
+      str.push_back(got);
+      got = is.rdbuf()->snextc();
     }
   }
   if (!extracted) {