clang compiler warnings: Fix pointer-bool-converesion warnings

clang compiler warnings: Fix pointer-bool-converesion warnings

This patch fixes several warnings pointed out by the clang compiler.
* app_minivm: Fixed evaluation of etemplate->locale, which will always
  evaluate to 'true'. This patch changes the evaluation to use
  ast_strlen_zero.
* app_queue:
  - Fixed evaluation of qe->parent->monfmt, which always evaluates to
    true. Instead, we just check to see if the dereferenced pointer
    evaluates to true.
  - Fixed evaluation of mem->state_interface, wrapping it with a call to
    ast_strlen_zero.

Review: https://reviewboard.asterisk.org/r/4541

ASTERISK-24917
Reported by: dkdegroot
patches:
  rb4541.patch submitted by dkdegroot (License 6600)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@434285 65c4cc65-6c06-0410-ace0-fbb531ad65f3