Initial part of manually colored prompt is colored incorrectly

[gwsw]

Steps to reproduce:
Run echo hello | ./less --use-color -R -Ps$(printf 'aa\e[41mbb\e[mcc')

Expected result:
Prompt should be "aabbcc" with the "aa" and "cc" uncolored, and the "bb" colored red.

Actual result:
"aa" is colored with the default prompt color instead of being uncolored.

Tested on less-643.

[gwsw]

Fixed in c33b98e.