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

Problem    : Result of operation is garbage or undefined @ 5809.
Diagnostic : Real issue.
Rationale  : When copying flags, first access to
             `use_pfxlist[use_pfxlen]` was garbage if
             `spin->si_compflags` was null.
Resolution : Make sure `use_pfxlist[use_pfxlen]` always has a value (NUL
             if `spin->si_compflags` is NULL).

src/nvim/spell.c

@@ -5804,6 +5804,8 @@ store_aff_word (
                   // Get compound IDS from the affix list.
                   get_compflags(affile, ae->ae_flags,
                       use_pfxlist + use_pfxlen);
+                else
+                  use_pfxlist[use_pfxlen] = NUL;
 
                 // Combine the list of compound flags.
                 // Concatenate them to the prefix IDs list.