avformat/dxa: Check fps to be within the supported range more precissely

Fixes: negation of -2147483648 cannot be represented in type 'int32_t' (aka 'int'); cast to an unsigned type to negate this value to itself
Fixes: assertion failure
Fixes: 29102/clusterfuzz-testcase-minimized-ffmpeg_dem_DXA_fuzzer-6744985740378112

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/dxa.c

@@ -79,7 +79,7 @@ static int dxa_read_header(AVFormatContext *s)
     if(fps > 0){
         den = 1000;
         num = fps;
-    }else if (fps < 0){
+    }else if (fps < 0 && fps > INT_MIN){
         den = 100000;
         num = -fps;
     }else{