Fix warnings: ex_docmd.c: eval_vars(): Unitialized arg: FP.

Problem    : Uninitialized argument value @ 7704.
Diagnostic : False positive.
Rationale  : Error occurs if `switch(spec_idx)` doesn't enter any case,
             which should not occur after
             `spec_idx = find_cmdline_var(...)` returned non-negative.
Resolution : Add default clause to switch and error if reached.

src/nvim/ex_docmd.c

@@ -7699,6 +7699,10 @@ eval_vars (
       sprintf((char *)strbuf, "%" PRId64, (int64_t)sourcing_lnum);
       result = strbuf;
       break;
+    default:
+      // should not happen
+      *errormsg = (char_u *)"";
+      return NULL;
     }
 
     resultlen = (int)STRLEN(result);            /* length of new string */