MdeModulePkg/HiiDatabaseDxe: Fix a variable is uninitialized.

MdeModulePkg/HiiDatabaseDxe: Fix a variable is uninitialized.

The StringSize variable in the FindStringBlock function is a IN OUT
parameter of the GetUnicodeStringTextOrSize function.
Thought the GetUnicodeStringTextOrSize function changes the value of
the StringSize variable, it is better to initialize the StringSize
variable before passing it to the GetUnicodeStringTextOrSize function.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Cinnamon Shia <cinnamon.shia@hpe.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>