Prevent text extractor's line cache from outliving the vcall to...

Prevent text extractor's line cache from outliving the vcall to performBlockWithLineCache, which caused a test to access freed memory

iTerm2.xcodeproj/project.pbxproj

@@ -8990,6 +8990,7 @@
 				);
 				GCC_GENERATE_DEBUGGING_SYMBOLS = YES;
 				GCC_OPTIMIZATION_LEVEL = 3;
+				"GCC_OPTIMIZATION_LEVEL[arch=*]" = 3;
 				GCC_VERSION = "";
 				GCC_WARN_ABOUT_MISSING_PROTOTYPES = YES;
 				GCC_WARN_FOUR_CHARACTER_CONSTANTS = NO;

sources/iTermTextExtractor.m

@@ -551,6 +551,7 @@ const NSInteger kLongMaximumWordLength = 100000;
     _shouldCacheLines = YES;
     block();
     _shouldCacheLines = NO;
+    _cachedLine = nil;
 }
 // Returns 0 if no value can be found less than or equal to `maximumValue`.