Unterminated #ifndef causes syntax highlighting to go wrong outside the source block

[JamesH65]

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If I have the following or similar, where the #ifndef is not terminated:

[source,c]
----
#ifndef HARDWARE_REGS_DEFINED
#define HARDWARE_REGS_DEFINED
----

Syntax highlighting in the rest of the file goes very wrong. Especially noticeable when I used ' (single quote, e.g. "it's") which causes large chunks of text to be incorrectly highlighted.