Fix the panic caused by .synth() when the VMOD is built with -O0.

A C beginner error was lurking here all this time.

For real now ...
Closes #2

src/vmod_file.c

@@ -604,7 +604,7 @@ vmod_reader_get(VRT_CTX, struct VPFX(file_reader) *rdr)
 VCL_VOID
 vmod_reader_synth(VRT_CTX, struct VPFX(file_reader) *rdr)
 {
-	const char *p[0];
+	const char *p[1];
 	struct strands strands = { 1, p };
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);