The crash during boot was caused by a DBUG_PRINT statement in fill_schema_schemata() (in
sql_show.cc). This DBUG_PRINT statement contained several instances of %s in the format 
string and for one of these we gave a NULL pointer as the argument. This caused the
call to vsnprintf() to crash when running on Solaris.
      
The fix for this problem is to replace the call to vsnprintf() with my_vsnprintf()
which handles that a NULL pointer is passed as argumens for %s.

This patch also extends my_vsnprintf() to support %i in the format string.

dbug/dbug.c:
  Replace the use of vsnprintf() with my_vsnprintf(). On some platforms
  vsnprintf() did not handle that a NULL pointer was given as an argument
  for a %s in the format string.
include/mysql/service_my_snprintf.h:
  Add support for %i in format string to my_vsnprintf().
strings/my_vsnprintf.c:
  Add support for %i in format string to my_vsnprintf().
unittest/mysys/my_vsnprintf-t.c:
  Add unit tests for %i in format string to my_vsnprintf().