vim-patch:8.2.4772: old Coverity warning for not checking ftell() return value

Problem:    Old Coverity warning for not checking ftell() return value.
Solution:   Check return value of fseek() and ftell().

3df8f6e353

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/errors.h

@@ -198,6 +198,7 @@ EXTERN const char e_failed_to_find_all_diff_anchors[] INIT( = N_("E1550: Failed
 EXTERN const char e_diff_anchors_with_hidden_windows[] INIT( = N_("E1562: Diff anchors cannot be used with hidden diff windows"));
 
 EXTERN const char e_trustfile[] INIT(= N_("E5570: Cannot update trust file: %s"));
+EXTERN const char e_cannot_read_from_str_2[] INIT(= N_("E282: Cannot read from \"%s\""));
 
 EXTERN const char e_unknown_option2[] INIT(= N_("E355: Unknown option: %s"));
 

src/nvim/main.c

@@ -2143,7 +2143,7 @@ static void source_startup_scripts(const mparm_T *const parmp)
       // Do nothing.
     } else {
       if (do_source(parmp->use_vimrc, false, DOSO_NONE, NULL) != OK) {
-        semsg(_("E282: Cannot read from \"%s\""), parmp->use_vimrc);
+        semsg(_(e_cannot_read_from_str_2), parmp->use_vimrc);
       }
     }
   } else if (!silent_mode) {

src/nvim/os/shell.c

@@ -789,15 +789,20 @@ char *get_cmd_output(char *cmd, char *infile, int flags, size_t *ret_len)
   // read the names from the file into memory
   FILE *fd = os_fopen(tempname, READBIN);
 
-  if (fd == NULL) {
+  // Not being able to seek means we can't read the file.
-    semsg(_(e_notopen), tempname);
+  long len_l;
+  if (fd == NULL
+      || fseek(fd, 0L, SEEK_END) == -1
+      || (len_l = ftell(fd)) == -1         // get size of temp file
+      || fseek(fd, 0L, SEEK_SET) == -1) {  // back to the start
+    semsg(_(e_cannot_read_from_str_2), tempname);
+    if (fd != NULL) {
+      fclose(fd);
+    }
     goto done;
   }
 
-  fseek(fd, 0, SEEK_END);
+  size_t len = (size_t)len_l;
-  size_t len = (size_t)ftell(fd);  // get size of temp file
-  fseek(fd, 0, SEEK_SET);
-
   buffer = xmalloc(len + 1);
   size_t i = fread(buffer, 1, len, fd);
   fclose(fd);