fortran: Fix conv of UNION constructors [PR105310]

This fixes an ICE when a UNION is the (1+8*2^n)-th field in a DEC
STRUCTURE when compiled with -finit-derived -finit-local-zero.
The problem was CONSTRUCTOR_APPEND_ELT from within gfc_conv_union_initializer
modified the vector pointer, but the pointer was passed by-value,
so the old pointer from the caller (gfc_conv_structure) pointed to freed
memory.

	PR fortran/105310

gcc/fortran/ChangeLog:

	* trans-expr.cc (gfc_conv_union_initializer): Pass vec* by reference.

gcc/testsuite/ChangeLog:

	* gfortran.dg/dec_union_12.f90: New test.

gcc/fortran/trans-expr.cc

@@ -9194,8 +9194,8 @@ gfc_trans_structure_assign (tree dest, gfc_expr * expr, bool init, bool coarray)
   return gfc_finish_block (&block);
 }
 
-void
-gfc_conv_union_initializer (vec<constructor_elt, va_gc> *v,
+static void
+gfc_conv_union_initializer (vec<constructor_elt, va_gc> *&v,
                             gfc_component *un, gfc_expr *init)
 {
   gfc_constructor *ctor;

gcc/testsuite/gfortran.dg/dec_union_12.f90

@@ -0,0 +1,43 @@
+! { dg-do compile }
+! { dg-options "-std=legacy -ffree-form -finit-local-zero -finit-derived -fdec-structure" }
+!
+! PR fortran/105310
+!
+! Test that gfc_conv_union_initializer does not cause an ICE when called
+! to build the constructor for a field which triggers a vector resize.
+!
+
+program dec_union_12
+  implicit none
+STRUCTURE /foo8u/
+  ! 8 fields
+  INTEGER(4) :: a,b,c,d,e,f,g,h
+  UNION
+  MAP
+  ENDMAP
+  ENDUNION
+ENDSTRUCTURE
+STRUCTURE /foo16u/
+  ! 16 fields
+  INTEGER(4) :: a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p
+  UNION
+  MAP
+  ENDMAP
+  ENDUNION
+ENDSTRUCTURE
+STRUCTURE /foo32u/
+  ! 32 fields
+  INTEGER(4) :: a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p
+  INTEGER(4) :: aa,ab,ac,ad,ae,af,ag,ah,ai,aj,ak,al,am,an,ao,ap
+  UNION
+  MAP
+  ENDMAP
+  ENDUNION
+ENDSTRUCTURE
+  record /foo8u/ bar8u
+  record /foo16u/ bar16u
+  record /foo32u/ bar32u
+  bar8u.a = 1
+  bar16u.a = 1
+  bar32u.a = 1
+end