Fix warnings: syntax.c: get_id_list(): Double free: FP.

Problem    : Double free @ 5213.
Diagnostic : False positive.
Rationale  : Suggested error path contains two consecutive invocations
             of `ends_excmd(*p)` having different results, which is not
             possible. First invocation is before the while loop. Second
             invocation is the while loop condition itsef.
Resolution : Refactor while loop into do-while loop. That removes the
             impossible path from analysis, and, in addition, is a bit
             more efficient.

src/nvim/ex_docmd.c

@@ -31,6 +31,7 @@
 #include "nvim/ex_getln.h"
 #include "nvim/fileio.h"
 #include "nvim/fold.h"
+#include "nvim/func_attr.h"
 #include "nvim/getchar.h"
 #include "nvim/hardcopy.h"
 #include "nvim/if_cscope.h"
@@ -4025,7 +4026,7 @@ static void ex_blast(exarg_T *eap)
   goto_buffer(eap, DOBUF_LAST, BACKWARD, 0);
 }
 
-int ends_excmd(int c)
+int ends_excmd(int c) FUNC_ATTR_CONST
 {
   return c == NUL || c == '|' || c == '"' || c == '\n';
 }