unbreak fbstring<wchar_t> default constructor (!!!)

Test Plan: folly/test/fbstring_test in debug mode

Reviewed By: sdoroshenko@fb.com

FB internal diff: D661622

folly/FBString.h

@@ -274,7 +274,7 @@ public:
   fbstring_core() {
     // Only initialize the tag, will set the MSBs (i.e. the small
     // string size) to zero too
-    ml_.capacity_ = maxSmallSize << (8 * (sizeof(size_t) - 1));
+    ml_.capacity_ = maxSmallSize << (8 * (sizeof(size_t) - sizeof(Char)));
     // or: setSmallSize(0);
     writeTerminator();
     assert(category() == isSmall && size() == 0);

folly/test/FBStringTest.cpp

@@ -993,6 +993,11 @@ TEST(FBString, testFixedBugs) {
       cp += "bb";
     }
   }
+  {
+    // D661622
+    basic_fbstring<wchar_t> s;
+    EXPECT_EQ(0, s.size());
+  }
 }