Fix an overflow found by ASAN.

Sometimes len is 0.

Change-Id: I031000188edc57663a5519b00cbbb751600d32f4

tools/aapt/Resource.cpp

@@ -553,7 +553,7 @@ static int validateAttr(const String8& path, const ResTable& table,
                     String8(parser.getElementName(&len)).string(), attr);
             return ATTR_LEADING_SPACES;
         }
-        if (str[len-1] == ' ') {
+        if (len != 0 && str[len-1] == ' ') {
             fprintf(stderr, "%s:%d: Tag <%s> attribute %s can not end with a space.\n",
                     path.string(), parser.getLineNumber(),
                     String8(parser.getElementName(&len)).string(), attr);