[flang][runtime] Initialize uninitialized pointer components

Pointer components without default initialization pose some
difficult (or impossible) problems when they appear as right-hand
side targets in pointer assignment statements; they may contain
garbage or stale data that looks enough like a valid descriptor
to cause a crash.  Solve the problem by avoiding it -- ensure
that pointers' descriptors are at least minimally established.

Differential Revision: https://reviews.llvm.org/D149979