[clang-format][PR41964] Fix crash with SIGFPE when TabWidth is set to 0 and line starts with tab

Summary:
clang-format 8.0 crashes with SIGFPE (floating point exception) when formatting following file:
app.cpp:
void a() {
	//line starts with '\t'
}

$ clang-format -style='{TabWidth: 0}' app.cpp

Reviewers: owenpan, klimek, russellmcc, timwoj

Reviewed By: klimek

Subscribers: cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D67670

llvm-svn: 372246