eval: port tv_get_buf_from_arg()

Cherry-picked from v8.2.1562, but uses Nvim's tv_check_str_or_nr().
Required for v8.2.1588.

It isn't used for f_bufnr() to avoid a double error message if the first
argument isn't a number or string similiar to what's seen in Vim.

src/nvim/eval/funcs.c

@@ -602,12 +602,7 @@ static void f_bufname(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   if (argvars[0].v_type == VAR_UNKNOWN) {
     buf = curbuf;
   } else {
-    if (!tv_check_str_or_nr(&argvars[0])) {
+    buf = tv_get_buf_from_arg(&argvars[0]);
-      return;
-    }
-    emsg_off++;
-    buf = tv_get_buf(&argvars[0], false);
-    emsg_off--;
   }
   if (buf != NULL && buf->b_fname != NULL) {
     rettv->vval.v_string = (char_u *)xstrdup((char *)buf->b_fname);
@@ -627,6 +622,9 @@ static void f_bufnr(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   if (argvars[0].v_type == VAR_UNKNOWN) {
     buf = curbuf;
   } else {
+    // Don't use tv_get_buf_from_arg(); we continue if the buffer wasn't found
+    // and the second argument isn't zero, but we want to return early if the
+    // first argument isn't a string or number so only one error is shown.
     if (!tv_check_str_or_nr(&argvars[0])) {
       return;
     }
@@ -653,18 +651,12 @@ static void f_bufnr(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 
 static void buf_win_common(typval_T *argvars, typval_T *rettv, bool get_nr)
 {
-  if (!tv_check_str_or_nr(&argvars[0])) {
+  const buf_T *const buf = tv_get_buf_from_arg(&argvars[0]);
+  if (buf == NULL) {  // no need to search if invalid arg or buffer not found
     rettv->vval.v_number = -1;
     return;
   }
 
-  emsg_off++;
-  buf_T *buf = tv_get_buf(&argvars[0], true);
-  if (buf == NULL) {  // no need to search if buffer was not found
-    rettv->vval.v_number = -1;
-    goto end;
-  }
-
   int winnr = 0;
   int winid;
   bool found_buf = false;
@@ -677,8 +669,6 @@ static void buf_win_common(typval_T *argvars, typval_T *rettv, bool get_nr)
     }
   }
   rettv->vval.v_number = (found_buf ? (get_nr ? winnr : winid) : -1);
-end:
-  emsg_off--;
 }
 
 /// "bufwinid(nr)" function
@@ -731,6 +721,18 @@ buf_T *tv_get_buf(typval_T *tv, int curtab_only)
   return buf;
 }
 
+/// Like tv_get_buf() but give an error message if the type is wrong.
+buf_T *tv_get_buf_from_arg(typval_T *const tv) FUNC_ATTR_NONNULL_ALL
+{
+  if (!tv_check_str_or_nr(tv)) {
+    return NULL;
+  }
+  emsg_off++;
+  buf_T *const buf = tv_get_buf(tv, false);
+  emsg_off--;
+  return buf;
+}
+
 /// Get the buffer from "arg" and give an error and return NULL if it is not
 /// valid.
 buf_T * get_buf_arg(typval_T *arg)
@@ -2799,15 +2801,11 @@ static void f_getbufinfo(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     }
   } else if (argvars[0].v_type != VAR_UNKNOWN) {
     // Information about one buffer.  Argument specifies the buffer
-    if (tv_check_num(&argvars[0])) {  // issue errmsg if type error
+    argbuf = tv_get_buf_from_arg(&argvars[0]);
-      emsg_off++;
-      argbuf = tv_get_buf(&argvars[0], false);
-      emsg_off--;
     if (argbuf == NULL) {
       return;
     }
   }
-  }
 
   // Return information about all the buffers or a specified buffer
   FOR_ALL_BUFFERS(buf) {
@@ -2875,13 +2873,7 @@ static void get_buffer_lines(buf_T *buf,
  */
 static void f_getbufline(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
-  buf_T *buf = NULL;
+  buf_T *const buf = tv_get_buf_from_arg(&argvars[0]);
-
-  if (tv_check_str_or_nr(&argvars[0])) {
-    emsg_off++;
-    buf = tv_get_buf(&argvars[0], false);
-    emsg_off--;
-  }
 
   const linenr_T lnum = tv_get_lnum_buf(&argvars[1], buf);
   const linenr_T end = (argvars[2].v_type == VAR_UNKNOWN