QTranslator: enable NRVO in find_translation() for poor compilers

... such as GCC.

truncate(0) was chosen because it is already used throughout
the function. Using clear(), say, which is inline, produces
slightly more code.

As is, saves 160b in text size on optimized GCC 4.9 Linux
AMD64 builds.

Change-Id: I415d09ad2b4547f1d69f78d85e2aa1c1f9a17ed3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>