Fix comparing wrong blocksize parameter variable

Compiler warning was unfortunately missed, but this
seems a clear case of comparing the wrong variable,
resulting in a condition which can never be true
(unsigned < 0).

A functional test is adapted to trigger the invalid
value case for `blockmaxsize` and check that the
message is produced.

Test Plan
---------

- `ninja all check-all`

src/init.cpp

@@ -1739,7 +1739,7 @@ bool AppInitParameterInteraction(Config &config) {
             return InitError(_("Cannot set both -blockmaxsize and -percentblockmaxsize"));
         } else if (has_bms && !has_pbms) {
             const int64_t nProposedMaxGeneratedBlockSize = gArgs.GetArg("-blockmaxsize", int64_t{-1});
-            if (nProposedExcessiveBlockSize < 0) {
+            if (nProposedMaxGeneratedBlockSize < 0) {
                 return InitError(_("Invalid value specified for -blockmaxsize"));
             }
             // Check blockmaxsize does not exceed maximum accepted block size.

test/functional/abc-cmdline.py

@@ -20,6 +20,8 @@
 MAX_GENERATED_BLOCK_SIZE_ERROR = (
     'Max generated block size (blockmaxsize) cannot exceed the excessive block size (excessiveblocksize)')
 
+INVALID_GENERATED_BLOCK_SIZE_ERROR = (
+    'Invalid value specified for -blockmaxsize')
 
 class ABC_CmdLine_Test (BitcoinTestFramework):
 
@@ -79,6 +81,10 @@ def excessiveblocksize_test(self):
         self.nodes[0].assert_start_raises_init_error(
             ['-blockmaxsize=1500000', '-excessiveblocksize=1300000'], 'Error: ' + MAX_GENERATED_BLOCK_SIZE_ERROR)
 
+        self.log.info("  Attempt to set invalid blockmaxsize (mining limit)")
+        self.nodes[0].assert_start_raises_init_error(
+            ['-blockmaxsize=-1'], 'Error: ' + INVALID_GENERATED_BLOCK_SIZE_ERROR)
+
         # Make sure we leave the test with a node running as this is what thee
         # framework expects.
         self.start_node(0, [])