String.IndexOf method handles negative start index value in incorrect way

The standard implementation of String.IndexOf converts the negative value of
the start index to 0 and searching will start from the beginning of the string.
But current implementation may start searching from the incorrect memory
offset, that can lead to sigsegv or return incorrect result.
This patch adds the handler for cases when fromIndex is negative.

Change-Id: I3ac86290712789559eaf5e46bef0006872395bfa
Signed-off-by: Alexei Zavjalov <alexei.zavjalov@intel.com>