[flang] Fix crash on erroneous program (#123843)

Catch and report multiple initializations of the same procedure pointer
rather than assuming that control wouldn't reach a given point in name
resolution in that case.

Fixes #123538.

Author: klausler

flang/lib/Semantics/resolve-names.cpp

@@ -8630,8 +8630,11 @@ void DeclarationVisitor::PointerInitialization(
     if (!context().HasError(ultimate)) {
       if (IsProcedurePointer(ultimate)) {
         auto &details{ultimate.get<ProcEntityDetails>()};
-        CHECK(!details.init());
-        if (const auto *targetName{std::get_if<parser::Name>(&target.u)}) {
+        if (details.init()) {
+          Say(name, "'%s' was previously initialized"_err_en_US);
+          context().SetError(ultimate);
+        } else if (const auto *targetName{
+                       std::get_if<parser::Name>(&target.u)}) {
           Walk(target);
           if (!CheckUseError(*targetName) && targetName->symbol) {
             // Validation is done in declaration checking.
@@ -8642,8 +8645,7 @@ void DeclarationVisitor::PointerInitialization(
         }
       } else {
         Say(name,
-            "'%s' is not a procedure pointer but is initialized "
-            "like one"_err_en_US);
+            "'%s' is not a procedure pointer but is initialized like one"_err_en_US);
         context().SetError(ultimate);
       }
     }

flang/test/Semantics/bug123538.f90

@@ -0,0 +1,7 @@
+!RUN: %python %S/test_errors.py %s %flang_fc1
+procedure(), pointer :: pp => tan
+!ERROR: EXTERNAL attribute was already specified on 'pp'
+!ERROR: POINTER attribute was already specified on 'pp'
+!ERROR: 'pp' was previously initialized
+procedure(real), pointer :: pp => tan
+end