[scan-build] fix warning emitted on Clang Driver code base

[apelete]

Summary

Fix a "logic error" warning of the type Called c++ object pointer is
null" reported by Clang Static Analyzer on the file:

• lib/Driver/Tools.cpp.

Signed-off-by: Apelete Seketeli <[e-mail]>

[dblaikie]

Should this be a test, or just an assertion?

lib/Driver/Tools.cpp

success =
         getAArch64MicroArchFeaturesFromMtune(D, A->getValue(), Args, Features);
   else if (success && (A = Args.getLastArg(options::OPT_mcpu_EQ)))
     success =
         getAArch64MicroArchFeaturesFromMcpu(D, A->getValue(), Args, Features);
   else if (Args.hasArg(options::OPT_arch))
     success = getAArch64MicroArchFeaturesFromMcpu(D, getAArch64TargetCPU(Args),
                                                   Args, Features);
 
-  if (!success)
+  if (!success && A)
     D.Diag(diag::err_drv_clang_unsupported) << A->getAsString(Args);
 
   if (Args.getLastArg(options::OPT_mgeneral_regs_only)) {
     Features.push_back("-fp-armv8");
     Features.push_back("-crypto");
     Features.push_back("-neon");
   }