Don't allow OS::SNPrint or OS::VSNPrint to return negative values.

vm/os.h

Index: vm/os.h
===================================================================
--- vm/os.h	(revision 9556)
+++ vm/os.h	(working copy)
@@ -62,16 +62,21 @@
   static void PrintErr(const char* format, ...);
   static void VFPrint(FILE* stream, const char* format, va_list args);
   // Print formatted output info a buffer.
+  //
   // Does not write more than size characters (including the trailing '\0').
-  // Returns the number of characters (excluding the trailing '\0') that would
-  // been written if the buffer had been big enough.
-  // If the return value is greater or equal than the given size then the output
-  // has been truncated.
-  // The buffer will always be terminated by a '\0', unless the buffer is of
-  // size 0.
-  // The buffer might be NULL if the size is 0.
-  // This specification conforms to C99 standard which is implemented by
-  // glibc 2.1+.
+  //
+  // Returns the number of characters (excluding the trailing '\0')
+  // that would been written if the buffer had been big enough.  If
+  // the return value is greater or equal than the given size then the
+  // output has been truncated.  The return value is never negative.
+  //
+  // The buffer will always be terminated by a '\0', unless the buffer
+  // is of size 0.  The buffer might be NULL if the size is 0.
+  //
+  // This specification conforms to C99 standard which is implemented
+  // by glibc 2.1+ with one exception: the C99 standard allows a
+  // negative return value.  We will terminate the vm rather than let
+  // that occur.
   static int SNPrint(char* str, size_t size, const char* format, ...);
   static int VSNPrint(char* str, size_t size,
                       const char* format,