Add asserts to avoid null pointer dereferences

Recently, several relvals in the 7_1_X IB's have segfaulted in RootInputFileSequence::readOneSpecified().
That function dereferences a pointer without first checking for a null pointer.
This function adds an assert to that function, and to a few other functions in the same source file, to always check  RootInputFileSequence::rootFile_, a pointer, for null before dereferencing it.
The segfaults are not reproducible, but this pull request may help solve the problem by checking for null pointer dereferences.
Some standardization of white space in the same file was also done, and one iterator loop was replaced by a C++11 range for loop.
range for loop.