Print useful error message when '\0' is used in a symbol name

AppendMacroArg() was passing 0 to sym_FindMacroArg(), which caused an assertion
failure. Now, AppendMacroArg() prints an error message instead.

test/asm/symbol-invalid-macro-arg.asm

@@ -0,0 +1 @@
+x\0

test/asm/symbol-invalid-macro-arg.out

@@ -0,0 +1,2 @@
+ERROR: symbol-invalid-macro-arg.asm(1):
+    Invalid macro argument '\0' in symbol