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

src/fast-dtoa.cc

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 684 matching lines @@
 bool FastDtoa(double v,
               FastDtoaMode mode,
               int requested_digits,
               Vector<char> buffer,
               int* length,
               int* decimal_point) {
   ASSERT(v > 0);
   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);
       break;
     case FAST_DTOA_PRECISION:
       result = Grisu3Counted(v, requested_digits,
                              buffer, length, &decimal_exponent);
       break;
+    default:
+      UNREACHABLE();
   }
   if (result) {
     *decimal_point = *length + decimal_exponent;
     buffer[*length] = '\0';
   }
   return result;
 }
 
 } }  // namespace v8::internal