Fix warnings: spell.c: spell_move_to(): Garbage value: RI.

Problem    : Result of operation is garbage or undefined @ 2238.
Diagnostic : Real issue.
Rationale  : Problem occurs when searching forward starting on an empty
             line. This is, at 2127:
             ```
             p = buf + skip;
             endp = buf + len;
             while (p < endp) {
             ```
             when skip == 0, len == 0, implying p == endp and therefore
             not entering the loop.
             Under those conditions, comparison
             ```
             if (attr == HLF_COUNT)
             ```
             at line 2242 is really using a garbage value for `attr`.
             Most of the time the error doesn't produce visible problems
             as it only affects when dealing with wrapped words.
Resolution : Initialize `attr` at declaration to `HLF_COUNT`, which is
             used in the code when no bad word found yet.

src/nvim/spell.c

@@ -2061,7 +2061,7 @@ spell_move_to (
   char_u      *line;
   char_u      *p;
   char_u      *endp;
-  hlf_T attr;
+  hlf_T attr = HLF_COUNT;
   int len;
   int has_syntax = syntax_present(wp);
   int col;