QWebpHandler: init m_webpData

Coverity complained (since 2016!) that m_webpData was left
uninitialized by the ctor. It's kind of a False Positive, because the
member is only used in ensureDemuxer(), but, as a low-risk fix for
picking back, just use NSDMI to value-initialize it.

Amends 1d4f24820c0fff474d524e006d715e13e409a4b8 (5.7).

Not picking to ESM branches, as it's clearly a FP.

Pick-to: 6.12 6.11 6.8
Coverity-Id: 22032
Change-Id: I79f422ff1f3285fb4364b951167f8cfff2c7c685
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>