JavaScriptMinifier: Fix "Uninitialized offset" in string and regexp parsing

JavaScriptMinifier: Fix "Uninitialized offset" in string and regexp parsing

When parsing an incomplete string literal or regex literal at the end of a file,
$end would be set to an offset higher than $length, because the code
speculatively increases $end without correcting for this scenario.

This is due to the assumption that the strcspn() search will end because
an end character was seen, instead of simply ending because the string
doesn't have any further characters.

Bug: T75556
Change-Id: I2325c9aff33293c13ff414699c2d47306182aaa6