Add some negative testcases for hexadecimal and octal parsing

tst/testinstall/strings.tst

@@ -49,6 +49,28 @@ gap> x:="\0x42\0x23\0x10\0x10\0x10";
 "B#\020\020\020"
 gap> x:="A string with \0xFF Hex stuff \0x42 in it";
 "A string with \377 Hex stuff B in it"
+gap> "\0yab";
+Syntax error: Expecting hexadecimal escape, or two more octal digits in stream\
+:1
+"\0yab";
+  ^
+gap> "\090";
+Syntax error: Expecting hexadecimal escape, or two more octal digits in stream\
+:1
+"\090";
+  ^
+gap> "\0x1g";
+Syntax error: Expecting hexadecimal digit in stream:1
+"\0x1g";
+    ^
+gap> '\0x1bc';
+Syntax error: Missing single quote in character constant in stream:1
+'\0x1bc';
+     ^
+Syntax error: ; expected in stream:2
+^
+gap> "\0x1bc";
+"\033c"
 
 # Empty string
 gap> x:="";