Fix failing tests, make expressions like this work: -i"F_MISSING>=1/5" (see also f4d7c27)

Author: pd3

filter.c

@@ -1366,7 +1366,6 @@ static void filters_set_alt_string(filter_t *flt, bcf1_t *line, token_t *tok)
 }
 static void filters_set_nmissing(filter_t *flt, bcf1_t *line, token_t *tok)
 {
-    if ( !tok->nsamples ) error("The function %s works with FORMAT fields\n", tok->tag);
     assert(tok->usmpl);
 
     bcf_unpack(line, BCF_UN_FMT);
@@ -1409,7 +1408,8 @@ static void filters_set_nmissing(filter_t *flt, bcf1_t *line, token_t *tok)
         default: fprintf(stderr,"todo: type %d\n", fmt->type); exit(1); break;
     }
     #undef BRANCH
-    tok->nvalues = 1;
+    tok->nsamples = 0;
+    tok->nvalues  = 1;
     tok->values[0] = tok->tag[0]=='N' ? nmissing : (nsmpl ? (double)nmissing / nsmpl : 0);
 }
 static int func_npass(filter_t *flt, bcf1_t *line, token_t *rtok, token_t **stack, int nstack)