Ensure we're not UNPROTECT()ing 'x' in gsumm (#6306)

* Ensure we're not UNPROTECT()ing 'x'

* UNPROTECT()+PROTECT() approach

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Co-authored-by: Benjamin Schwendinger <52290390+ben-schwen@users.noreply.github.com>

src/gsumm.c

@@ -594,8 +594,9 @@ SEXP gmean(SEXP x, SEXP narmArg)
     x = PROTECT(coerceVector(x, REALSXP)); protecti++;
   case REALSXP: {
     if (INHERITS(x, char_integer64)) {
-      x = PROTECT(coerceAs(x, /*as=*/PROTECT(ScalarReal(1)), /*copyArg=*/ScalarLogical(TRUE))); protecti++;
-      UNPROTECT(1); // as= input to coerceAs()
+      SEXP as = PROTECT(ScalarReal(1));
+      x = PROTECT(coerceAs(x, as, /*copyArg=*/ScalarLogical(TRUE))); protecti++;
+      UNPROTECT(2); PROTECT(x); // PROTECT() is stack-based, UNPROTECT() back to 'as' then PROTECT() 'x' again
     }
     const double *restrict gx = gather(x, &anyNA);
     ans = PROTECT(allocVector(REALSXP, ngrp)); protecti++;