Fix build-breakage on Mac. Initial variable in unreachable branch.

src/fast-dtoa.cc

Index: src/fast-dtoa.cc
===================================================================
--- src/fast-dtoa.cc	(revision 5491)
+++ src/fast-dtoa.cc	(working copy)
@@ -702,7 +702,7 @@
   ASSERT(!Double(v).IsSpecial());
 
   bool result = false;
-  int decimal_exponent;
+  int decimal_exponent = 0;
   switch (mode) {
     case FAST_DTOA_SHORTEST:
       result = Grisu3(v, buffer, length, &decimal_exponent);
@@ -711,6 +711,8 @@
       result = Grisu3Counted(v, requested_digits,
                              buffer, length, &decimal_exponent);
       break;
+    default:
+      UNREACHABLE();
   }
   if (result) {
     *decimal_point = *length + decimal_exponent;