Address some leftover review comments from previous CL.

vm/os_macos.cc

Index: vm/os_macos.cc
===================================================================
--- vm/os_macos.cc	(revision 9676)
+++ vm/os_macos.cc	(working copy)
@@ -136,26 +136,25 @@
 }
 
 
-bool OS::StringToInteger(const char* str, int64_t* value) {
+bool OS::StringToInt64(const char* str, int64_t* value) {
   ASSERT(str != NULL && strlen(str) > 0 && value != NULL);
-  bool negative_value = false;
   int32_t base = 10;
+  char* endptr;
+  int i = 0;
   if (str[0] == '-') {
-    negative_value = true;
-    str += 1;
+    i = 1;
   }
-  if ((str[0] == '0') && (str[1] == 'x' || str[1] == 'X') && (str[2] != '\0')) {
+  if ((str[i] == '0') &&
+      (str[i + 1] == 'x' || str[i + 1] == 'X') &&
+      (str[i + 2] != '\0')) {
     base = 16;
   }
   errno = 0;
-  *value = strtoll(str, NULL, base);
-  if (errno == 0) {
-    if (negative_value) {
-      *value = -(*value);
-    }
-    return true;
+  *value = strtoll(str, &endptr, base);
+  if (errno != 0 || endptr == str || *endptr != 0) {
+    return false;
   }
-  return false;
+  return true;
 }