libstagefright: fix indefinate loop in QueryCodec

portFormat.nIndex is being incremented which is not
trustworthy since the nIndex value could be overriden
by the OMX Component,  which causes an indefinate loop
which inturn causes a memory leak and crashes the system.

OMX Component on encore and p970 exhibits this behaviour
(OMX.TI.720P.Decoder). This patch prevents stagefright
freezes when QueryCodec is called during Gallery Thumbnail
generation for videos and Adobe Flash playback.

Change-Id: I825c99ddecacbb927e22ac7d1a53facb26d95ff2