Fix crash in app_minivm when mime encoding string

An incorrect string initializations was left in ast_str_encode_mime from the
patch that converted string manipulations to use ast_str strings (r191140).
The string initialization causes a crash when ast_str_set is called on
the string later on in the function.

(closes issue ASTERISK-18967)
Reported by: Chris Boot
patches:
  minivm-null-pointer-dereference-fix.patch uploaded by bootc (license 6309)

(issue ASTERISK-20854)
Reported by: Chris Warr
Tested by: Chris Warr
........

Merged revisions 379608 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@379609 f38db490-d61c-443f-a65b-d21fe96a405b