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Enable GCC options to detect undefined behaviour

GCC has an Undefined Behavior Sanitizer (ubsan), which enables run-time
checks of undefined behaviour. It has been enabled for the `develop`
build target.

A small bug detected with it has been fixed.

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

--- a/Makefile

+++ b/Makefile

@@ -207,7 +207,12 @@

 		-Wstringop-overflow=4 -Walloc-zero -Wduplicated-cond \

 		-Wfloat-equal -Wshadow -Wcast-qual -Wcast-align -Wlogical-op \

 		-Wnested-externs -Wno-aggressive-loop-optimizations -Winline \

-		-Wundef -Wstrict-prototypes -Wold-style-definition"

+		-Wundef -Wstrict-prototypes -Wold-style-definition \

+		-fsanitize=shift -fsanitize=integer-divide-by-zero \

+		-fsanitize=unreachable -fsanitize=vla-bound \

+		-fsanitize=signed-integer-overflow -fsanitize=bounds \

+		-fsanitize=object-size -fsanitize=bool -fsanitize=enum \

+		-fsanitize=alignment -fsanitize=null"

 # Targets for the project maintainer to easily create Windows exes.

 # This is not for Windows users!

--- a/src/link/object.c

+++ b/src/link/object.c

@@ -36,14 +36,14 @@

  */

 static int32_t readlong(FILE *f)

 {

-	int32_t r;

+	uint32_t r;

-	r = fgetc(f);

-	r |= fgetc(f) << 8;

-	r |= fgetc(f) << 16;

-	r |= fgetc(f) << 24;

+	r = ((uint32_t)(uint8_t)fgetc(f));

+	r |= ((uint32_t)(uint8_t)fgetc(f)) << 8;

+	r |= ((uint32_t)(uint8_t)fgetc(f)) << 16;

+	r |= ((uint32_t)(uint8_t)fgetc(f)) << 24;

-	return r;

+	return (int32_t)r;

 }

 /*