Fix possible unterminated string

sym_SetMacroArgID used a `sprintf` that could write no \0.
In practice this was benign because %u cannot print 256 chars,
but better future-proof this.

src/asm/symbol.c

@@ -440,7 +440,7 @@ void sym_SetMacroArgID(uint32_t nMacroCount)
 {
 	char s[256];
 
-	snprintf(s, sizeof(s), "_%u", nMacroCount);
+	snprintf(s, sizeof(s) - 1, "_%u", nMacroCount);
 	newmacroargs[MAXMACROARGS] = strdup(s);
 }