Fix assertion in light tree diagnostic reporter

happened then two diagnostics are reported on the same element.
#KT-55924 fixed

compiler/cli/src/org/jetbrains/kotlin/cli/common/fir/FirDiagnosticsCompilerResultsReporter.kt

@@ -162,10 +162,10 @@ private class SequentialFilePositionFinder(file: File) : Closeable {
 
     // assuming that if called multiple times, calls should be sorted by ascending offset
     fun findNextPosition(offset: Int, withLineContents: Boolean = true): KtSourceFilePos {
-        assert(offset >= charsRead - (currentLineContent?.length ?: 0))
 
         fun posInCurrentLine(): KtSourceFilePos? {
             val col = offset - (charsRead - currentLineContent!!.length - 1)/* beginning of line offset */ + 1 /* col is 1-based */
+            assert(col > 0)
             return if (col <= currentLineContent!!.length)
                 KtSourceFilePos(currentLine, col, if (withLineContents) currentLineContent else null)
             else null

compiler/testData/cli/jvm/twoDiagnosticsOnSingleElement.args

@@ -0,0 +1,6 @@
+-Xuse-fir-lt=true
+-language-version
+2.0
+$TESTDATA_DIR$/twoDiagnosticsOnSingleElement.kt
+-d
+$TEMP_DIR$

compiler/testData/cli/jvm/twoDiagnosticsOnSingleElement.kt

@@ -0,0 +1,11 @@
+package test
+
+interface I1 {
+    fun <T> foo()
+}
+
+interface I2 {
+    fun foo()
+}
+
+class C : I1, I2

compiler/testData/cli/jvm/twoDiagnosticsOnSingleElement.out

@@ -0,0 +1,8 @@
+warning: language version 2.0 is experimental, there are no backwards compatibility guarantees for new language and library features
+compiler/testData/cli/jvm/twoDiagnosticsOnSingleElement.kt:11:1: error: class C is not abstract and does not implement abstract member foo
+class C : I1, I2
+^
+compiler/testData/cli/jvm/twoDiagnosticsOnSingleElement.kt:11:1: error: c inherits conflicting members: foo, foo
+class C : I1, I2
+^
+COMPILATION_ERROR

compiler/tests-gen/org/jetbrains/kotlin/cli/CliTestGenerated.java

@@ -1077,6 +1077,11 @@ public class CliTestGenerated extends AbstractCliTest {
             runTest("compiler/testData/cli/jvm/syntheticAccessorSignatureClash.args");
         }
 
+        @TestMetadata("twoDiagnosticsOnSingleElement.args")
+        public void testTwoDiagnosticsOnSingleElement() throws Exception {
+            runTest("compiler/testData/cli/jvm/twoDiagnosticsOnSingleElement.args");
+        }
+
         @TestMetadata("unknownExtraFlags.args")
         public void testUnknownExtraFlags() throws Exception {
             runTest("compiler/testData/cli/jvm/unknownExtraFlags.args");