Avoid out of bounds access in String::fromCharCode (#1254)

If c is negative (this can happen if char is signed), then accessing
sConstStrings[c] is an out of bounds memory access. This results in a
string being created from garbage memory, which can lead to seg faults
later on.

Taking the other branch is safer in this case.

Author: tobil4sk

src/String.cpp

@@ -1380,7 +1380,7 @@ Dynamic String::charCodeAt(int inPos) const
 
 String String::fromCharCode( int c )
 {
-   if (c<=255)
+   if (0<=c && c<=255)
    {
       return sConstStrings[c];
    }