[MC] Don't crash when trying to emit a relocation against .bss.

Turn that into an error instead.

llvm-svn: 276783
diff --git a/llvm/lib/MC/MCAssembler.cpp b/llvm/lib/MC/MCAssembler.cpp
index 7a42108..4735f6c 100644
--- a/llvm/lib/MC/MCAssembler.cpp
+++ b/llvm/lib/MC/MCAssembler.cpp
@@ -575,8 +575,8 @@
         // into a virtual section. This is to support clients which use standard
         // directives to fill the contents of virtual sections.
         const MCDataFragment &DF = cast<MCDataFragment>(F);
-        assert(DF.fixup_begin() == DF.fixup_end() &&
-               "Cannot have fixups in virtual section!");
+        if (DF.fixup_begin() != DF.fixup_end())
+          report_fatal_error("cannot have fixups in virtual section!");
         for (unsigned i = 0, e = DF.getContents().size(); i != e; ++i)
           if (DF.getContents()[i]) {
             if (auto *ELFSec = dyn_cast<const MCSectionELF>(Sec))