lib: qmi_encdec: Fix initialization error in string encode/decode

The string length variable in string encode/decode function is not
initialized to zero. It may result in decoding failure in case it is
initialized to some garbage value from stack.

Initialize the variables to zero to avoid decoding failure.

Change-Id: I77c8dca296f1070f7bd7e6b79c7dc898c3fc23b4
Signed-off-by: Atish Kumar Patra <apatra@codeaurora.org>