coverity/13745: Argument cannot be negative: RI.

Problem    : Argument cannot be negative @ 1165.
Diagnostic : Real issue.
Rationale  : len can be assigned a negative value @ 1162;
             len is passed as an unsigned argument @ 1165.
Resolution : Refactor variable's types:
             - Use ftello instead of ftell to avoid using long.
             - Assert ftello result is safely convertible to size_t.
             - Introduce variable read_size to avoid using i (int).

config/CMakeLists.txt

@@ -7,6 +7,8 @@ check_type_size("int" SIZEOF_INT)
 check_type_size("long" SIZEOF_LONG)
 check_type_size("intmax_t" SIZEOF_INTMAX_T)
 check_type_size("off_t" SIZEOF_OFF_T)
+check_type_size("size_t" SIZEOF_SIZE_T)
+check_type_size("long long" SIZEOF_LONG_LONG)
 check_type_size("void *" SIZEOF_VOID_PTR)
 
 check_symbol_exists(_NSGetEnviron crt_externs.h HAVE__NSGETENVIRON)

src/nvim/os_unix.c

@@ -1159,16 +1159,30 @@ int mch_expand_wildcards(int num_pat, char_u **pat, int *num_file,
     free(tempname);
     goto notfound;
   }
-  fseek(fd, 0L, SEEK_END);
+  int fseek_res = fseek(fd, 0L, SEEK_END);
-  len = ftell(fd);                      /* get size of temp file */
+  if (fseek_res < 0) {
+    free(tempname);
+    fclose(fd);
+    return FAIL;
+  }
+  long long templen = ftell(fd);        /* get size of temp file */
+  if (templen < 0) {
+    free(tempname);
+    fclose(fd);
+    return FAIL;
+  }
+#if SIZEOF_LONG_LONG > SIZEOF_SIZE_T
+  assert(templen <= (long long)SIZE_MAX);
+#endif
+  len = (size_t)templen;
   fseek(fd, 0L, SEEK_SET);
   buffer = xmalloc(len + 1);
   // fread() doesn't terminate buffer with NUL;
   // appropiate termination (not always NUL) is done below.
-  i = fread((char *)buffer, 1, len, fd);
+  size_t readlen = fread((char *)buffer, 1, len, fd);
   fclose(fd);
   os_remove((char *)tempname);
-  if (i != (int)len) {
+  if (readlen != len) {
     /* unexpected read error */
     EMSG2(_(e_notread), tempname);
     free(tempname);