[media][sfplugin] fix -Wdangling-gsl

C2BufferData#linearBlocks() returns a std::vector. Chaining method calls
results in an immediate dereference of a dangling pointer. Save a
reference to the otherwise temporary for the duration of the function
call.

Bug: 142544793
Test: m, flash device
Change-Id: I99d662a8fe5deec652cdcd82f420c836c76d5539
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>