Fix warnings: edit.c: ins_compl_get_exp(): Np dereference (2): FP.

Problems   : Dereference of null pointer @ 3615.
             Dereference of null pointer @ 3764.
Diagnostic : False positives.
Rationale  : `ins_buf` is local static, so maintains value between calls.
             This function will be called first when `compl_started` is
             false, and in that case it initializes `ins_buf`. After
             that, it can be called multiple times with `compl_started`
             true, where `ins_buf` will be updated but not to null.
             So, when arriving to both points, `ins_buf` should never be
             null.
Resolution : Assert `ins_buf` at both points.

src/nvim/edit.c

@@ -3613,6 +3613,7 @@ static int ins_compl_get_exp(pos_T *ini)
        * If 'infercase' is set, don't use 'smartcase' here
        */
       save_p_scs = p_scs;
+      assert(ins_buf);
       if (ins_buf->b_p_inf)
         p_scs = FALSE;
 
@@ -3761,8 +3762,10 @@ static int ins_compl_get_exp(pos_T *ini)
       compl_started = TRUE;
     } else {
       /* Mark a buffer scanned when it has been scanned completely */
-      if (type == 0 || type == CTRL_X_PATH_PATTERNS)
+      if (type == 0 || type == CTRL_X_PATH_PATTERNS) {
+        assert(ins_buf);
         ins_buf->b_scanned = TRUE;
+      }
 
       compl_started = FALSE;
     }