[gdb/symtab] Make find_block_in_blockvector more robust

While working on PR25858 I noticed that the following trigger patch:
...
@@ -2918,6 +2918,7 @@ find_pc_sect_compunit_symtab
          const struct blockvector *bv;

          bv = COMPUNIT_BLOCKVECTOR (cust);
+         (volatile int)blockvector_contains_pc (bv, pc);
          b = BLOCKVECTOR_BLOCK (bv, GLOBAL_BLOCK);

          if (BLOCK_START (b) <= pc
...
triggers this assert, which checks that the returned block indeed
contains pc:
...
@@ -170,7 +170,10 @@ find_block_in_blockvector

     {
       b = BLOCKVECTOR_BLOCK (bl, bot);
       if (BLOCK_END (b) > pc)
-       return b;
+       {
+         gdb_assert (BLOCK_START (b) <= pc);
+         return b;
+       }
       bot--;
     }

...
when running test-case gdb.ada/bp_c_mixed_case.exp.

It's possible that the trigger patch breaks an undocumented invariant:  I've
tried a build and test run without the trigger patch and did not manage to
trigger the assert.

For robustness-sake, fix the assert by bailing out if 'BLOCK_START (b) <= pc'
doesn't hold.

Tested on x86_64-linux.

gdb/ChangeLog:

2020-10-22  Tom de Vries  <tdevries@suse.de>

	* block.c (find_block_in_blockvector): Make sure the returned block
	contains pc.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-10-22  Tom de Vries  <tdevries@suse.de>
+
+	* block.c (find_block_in_blockvector): Make sure the returned block
+	contains pc.
+
 2020-10-22  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	PR gdb/26693

gdb/block.c

@@ -166,6 +166,8 @@ find_block_in_blockvector (const struct blockvector *bl, CORE_ADDR pc)
   while (bot >= STATIC_BLOCK)
     {
       b = BLOCKVECTOR_BLOCK (bl, bot);
+      if (!(BLOCK_START (b) <= pc))
+	return NULL;
       if (BLOCK_END (b) > pc)
 	return b;
       bot--;