Fix unicode compiler warning

The compiler warned about the unicode_to_utf8 code.
I fixed it by giving an error. This is the same as gcc does.

tccpp.c

@@ -352,6 +352,7 @@ ST_INLN char *unicode_to_utf8 (char *b, uint32_t Uc)
     else if (Uc-0xd800u<0x800) return b;
     else if (Uc<0x10000) *b++=224+Uc/4096, *b++=128+Uc/64%64, *b++=128+Uc%64;
     else if (Uc<0x110000) *b++=240+Uc/262144, *b++=128+Uc/4096%64, *b++=128+Uc/64%64, *b++=128+Uc%64;
+    else tcc_error("0x%x is not a valid universal character", Uc);
     return b;
 }
 

tests/tests2/60_errors_and_warnings.c

@@ -442,4 +442,10 @@ int f() { ({ return 78; }); }
 int main() { return f(); }
 
 /******************************************************************/
+
+#elif defined test_illegal_unicode
+int main() {
+    char *str = "\Uffffffff";
+}
+
 #endif

tests/tests2/60_errors_and_warnings.expect

@@ -216,3 +216,6 @@ arg[1] = "Y"
 
 [test_return_from_statement_expr]
 [returns 78]
+
+[test_illegal_unicode]
+60_errors_and_warnings.c:448: error: 0xffffffff is not a valid universal character