Sema: Revert sema.err to null if the Decl already has an error

Previously we would assign the error message to Sema and then never
clear it even when destroying the error message, which caused memory
corruption.

Closes #12437

Author: sin-ack

src/Sema.zig

@@ -1906,8 +1906,6 @@ fn failWithOwnedErrorMsg(sema: *Sema, err_msg: *Module.ErrorMsg) CompileError {
     }
 
     const mod = sema.mod;
-    sema.err = err_msg;
-
     {
         errdefer err_msg.destroy(mod.gpa);
         if (err_msg.src_loc.lazy == .unneeded) {
@@ -1925,8 +1923,10 @@ fn failWithOwnedErrorMsg(sema: *Sema, err_msg: *Module.ErrorMsg) CompileError {
     const gop = mod.failed_decls.getOrPutAssumeCapacity(sema.owner_decl_index);
     if (gop.found_existing) {
         // If there are multiple errors for the same Decl, prefer the first one added.
+        sema.err = null;
         err_msg.destroy(mod.gpa);
     } else {
+        sema.err = err_msg;
         gop.value_ptr.* = err_msg;
     }
     return error.AnalysisFail;