DexDump: Leak in FieldMethodInfo.signature

Free pMethInfo->signature when filled by getMethodInfo.

Change-Id: Id278d732374daceb7b8b05be7f433af25eee0c27
Fix DDMS crash

(cherry-pick of 22d8dba8ae5be7aecb07322a410712206896fa15)

Change-Id: Ic5edd3d9367db8570523e1380c3e6edfabeec2c0