Fix warnings: edit.c: ins_compl_next_buf(): Np dereference: MI.

Problem    : Dereference of null pointer @ 3247.
Diagnostic : Multithreading issue.
Rationale  : Problem only occurs if global `ctrl_x_mode` is modified
             while calling function is executing.
Solution   : Use local copy instead of global.
Fix warnings: message.c: msg_attr_keep(): Np dereference: FP.

Problem    : Dereference of null pointer @ 179.
Diagnostic : False positive.
Rationale  : Error occurs if `s` paramater is null, which should not
             happen.
Resolution : Declare parameter as non null.
Merge pull request #1618 from elmart/clang-analysis-fixes-5

Fix clang analysis warnings. (5)