Allow detection of one char after the C-* is detected

[seekshiva]

While detecting the C-* shortcuts, the first character after detecting an invalid shortcut must be detected as a part of the shortcut sequence and checked against the list fo available shortcuts. For example when the user types the following sequence of characters:

a s d f C-X b o y

the characters must be detected as

a s d f [C-X b] o y

and the [C-X b] shortcut must be acted upon. but when the user types the following sequence:

a s d f C-X C-B b o y

the characters must be detected as

a s d f [C-X C-B] b o y

and not

a s d f [C-X C-B b] o y

i.e., emacs must stop detection of the shortcut (and adding it to the minibuffer) once a valid shortcut sequence(in this case, C-X C-B) is detected.