Fix "first code unit" bug.

src/pcre2_compile.c

@@ -6750,14 +6750,15 @@ for (;;)
       we have to abandon the firstcu for the regex, but if there was
       previously no reqcu, it takes on the value of the old firstcu. */
 
-      if (firstcuflags >= 0 &&
-         (firstcuflags != branchfirstcuflags ||
-          firstcu != branchfirstcu))
+      if (firstcuflags != branchfirstcuflags || firstcu != branchfirstcu)
         {
-        if (reqcuflags < 0)
+        if (firstcuflags >= 0)
           {
-          reqcu = firstcu;
-          reqcuflags = firstcuflags;
+          if (reqcuflags < 0)
+            {
+            reqcu = firstcu;
+            reqcuflags = firstcuflags;
+            }
           }   
         firstcuflags = REQ_NONE;
         }

testdata/testinput1

@@ -5698,4 +5698,8 @@ name)/mark
     A8B
     A9B  
 
+/(|ab)*?d/
+   abd
+   xyd 
+
 # End of testinput1 

testdata/testinput2

@@ -3995,4 +3995,12 @@ a random value. /Ix
 
 /abcd/I,no_start_optimize
 
+/(|ab)*?d/I
+   abd
+   xyd 
+
+/(|ab)*?d/I,no_start_optimize
+   abd
+   xyd 
+
 # End of testinput2 

testdata/testoutput1

@@ -9396,4 +9396,11 @@ No match
     A9B  
  0: A9B
 
+/(|ab)*?d/
+   abd
+ 0: abd
+ 1: ab
+   xyd 
+ 0: d
+
 # End of testinput1 

testdata/testoutput2

@@ -13554,4 +13554,26 @@ Capturing subpattern count = 0
 Options: no_start_optimize
 Subject length lower bound = 0
 
+/(|ab)*?d/I
+Capturing subpattern count = 1
+Starting code units: a d 
+Last code unit = 'd'
+Subject length lower bound = 1
+   abd
+ 0: abd
+ 1: ab
+   xyd 
+ 0: d
+
+/(|ab)*?d/I,no_start_optimize
+Capturing subpattern count = 1
+Options: no_start_optimize
+Last code unit = 'd'
+Subject length lower bound = 0
+   abd
+ 0: abd
+ 1: ab
+   xyd 
+ 0: d
+
 # End of testinput2